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glass. In his paper on Telescope Construction, LETTERS TO THE EDITOR. I note the wisdom shown by Mr. Lancaster in not giving numerical values for the curves; I wish, though, that he had given a table of the specific gravity and dispersive and refractive powers of the commoner kinds of optical glass.

We do not hold ourselors responsible for the opinions of our correspondents. The Editor respectfully requests that ali communications should be drawn up as briefly as possible,] All communications should be addressed to the EDITOR of the ENGLISH MECHANIC, 31, Tavistock-strest, Covent-yaiden,

W.0.

All Cheques and Post-office Orders to be made Payable to J. PASSMORE EDWARDS.

In order to facilitate reference, Correspondents, when speaking of any Letter previously inserted, will oblige by mentioning the number of the Letter, as well as the page or which it appears.

"I would have everyone write what he knows, and ai much as he knows, but no more; and that not in this only, but in all other subjects: For such a person may have some particular knowledge and experience of the nature of such a person or such a fountain, that as to other things, knows no more than what everybody does, and yet, to keep a clutter with this Little gittance of his, will undertake to write the whole body of physicks: a vice from whence great inconveniences derive their original." -Montaigne's Essays.

CHEAP EQUATORIAL CLOCK-MOTIONS.

[18011.]-I HAVE been studying this question in your pages, and find a number of ingenious and efficient devices.

Bladders filled with air, where escape is regulated (a variation of the bellows governor, a French invention), and which may be replaced by the children's indiarubber balloons, less liable to attacks of mice; clepsydra, in which water regulates the descent of a weight.

world. I see, by your Notices, that an authority
eminent in these matters has been removed from
among us by death. An early and earnest worker,
he continued so to the last, and his paper on Work-
ing Specula is unequalled, in so far as regards the
special subject on which it treats. Though b
worked with metallic specula, bis instructions are
none the less valuable, and his name, together with
that of the Rev. Cooper Key, will for ever be as-
sociated with the history and development of the
reflecting telescope.

I do not profess to be an adept in the abstruse
subject of mathematical optics; but I will try to
explain Coddington's method as clearly as I pos-
sibly can. He does not treat of aberration in the
same manner as Barlow, aud, as several prelimi-
HERSCHEL'S TAPLE OF RADII OF THE SURFACES OF AN AHANATIC OBJECT-GLARS.
Crown Refractive Index = 1521: Flint Refractive Index 1585: Comp. Focus 1000.

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CONTINUATION OF THE TABLE OF CURVES, COMPUTED FROM BARLOW's RULES.
Object-glass, composed of Double Convex and Plano-Concave Lenses.

-.0'55. Crown Focus = 450 Comp. focus
Flint Focus 818'18) -- 1000.

I have not, however, seen published Mr. Lewis Swift's, Rochester, equatorial motor. It depends upon the uniformity of escape of sand through an orifice in a closed vessel. Dry sand flows out of a hole, with a velocity and in quantities independent of height of column of sand, or of weight resting on its surface; a vessel of cylindric or rectangular form, filled with fine dry sand, supports A. the surface of the sand, a suitable weight, 20 to 100lb., or more if desirable. The outflow of sand is regulated by a gate, so that the surface shall descend with the needed velocity; a cord for the weights, wrapped around a wheel, attached to a radius-bar fixed on the polar axis, gives the desired hourly motion.

A weighted piston, moving in a cylinder filled with water, the escape from below to above the piston regulated by a stop-cock, would also answer, but would be liable to freeze, and probably would not be so uniform in its action as the weight resting on the descending surface of dry sand.

With this sand c'ep-ydra, many of Prof. Swift's discoveries of double stars were made. Washington, U.S.A., Oct. 9.

Gimes.

Crown Focus Positive.

Flint Fucus Negative.

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99-62

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THE ACHROMATIC OBJECT-GLASS. [18012.]-ALTHOUGH I am ready and willing to nary letters are necessary, I think it best to send do what I can to assist our excellent friend on the one, giving a short explanation of several elementother side of the Atlantic, "Nova Scotian," Iary terms, which should be familiar to all optical must really decline to ask you, Mr. Editor, to fill two or three columns with numerical tables, which anyone who has thoroughly mastered Barlow's formula may compute for himself. Moreover, several of the tables are incomplete. I might possibly consider your correspondent rather unreasonable in his requests for mathematical formule, but he has so ably and so readily detailed his experience in object-glass making, and shows such a thorough interest in the subject, that I should be very sorry indeed to see any of his requests uncomplied with. Trusting he will send what help he can, in matters practical, I will try to do the same in matters theoretical: we shall thus be working for one common end, and shall, I trust, be of some slight service to many brother-read- rs.

Will "Nova Scotian "give us his experience in respect to facetted or graduated tools with varioussized graduations? Does Holtzapffel give any instructions for making micrometers and graduating equatorial circles?

I send a copy of a table, extracted from Herschel on "The Telescope," several years ago; and also a continuation of my table of curve values; in another letter, I will give the remaining portion. As regards sending an account of Coddington's formulæ, I fear some of those "rule of thumb " people who are not well up in "good going Greek" will be grumbling. But, if you, Sir, can spare the space, I will try to write a few letters, for, upon one point I am quite certain, and "Nova Scotian " will, no doubt, agree with me, that no amateur can ever hope to make a decent glass, unless his foci and radii are calculated from data obtained from the glass he intends to employ; any other course will end in disappointment. It will not do to copy curves merely because in a certain instrument they produce good effect. Chance's best discs are, I believe, uniform in quality, and, therefcre, the radii used, if correct in one case, will also be so in another; but, I imagine, few amateurs will care to experiment upon expensive discs of

833

ALTAZIMUTH

ΤΟ "F.R.A.S."-TRAIN FRICTION. TO

I was 80

I think "J. L." must be either asleep or izdalging in a mild joke.

"J. L." reply about the Altazimuth Equatorial. [18013.]-I WAS very glad to read "F.R.A.S.'s" fully convinced that Lord Lyndsay must be right, and that it was my gross stupidity that was in fault, that I have wasted sheet after sheet of paper in the endeavour to find out where my error lay. I think that not only is the elliptic disc untrue, ba I am sorry that none of our artist opticians can be induced to give a little information, for surely it circumstances. Moreover, owing to its variation in that no form of disc would work truly der es is a positive loss to science that the results of a life-position considered as a common tangent to the two long experience should be unrecorded. We do not dises, the connecting band would not transfer quite ask for "trade secrets"; we require hints as to the same peripheral motion to the driven disc that what direction we must pursue, in order that suc- it receives from the driver. At first, I thought this cess may reasonably be expected. No amount of might form a compensatory adjustment; but I earwritten instruction will enable one to make a per-not find that it would do so. fect object-glass, auy more than it will teach him to grind a perfect speculum. One great "secret," doubtless, is hard, unremitting work, a ceaseless devotion to the cause in hand, combined with adequate mathematical knowledge, to deduce the best curves for any particular glass. But Dallmeyer, Wray, and other great artists, must have tried and cast aside many and varied formule, and experimented upon numberless forms of object lenses, during their long and successful career. Surely they could not object to give us some account ef their past experience-past failures, if I may be allowed so to term it, for failures often bring out many important facts. I feel certain they might do this without detracting one iota from the proud position in which their skill and industry have placed them. I understand Mr. Wray did, at one time, occasionally contribute to these columns; let me hope he will do so again.

On the silvered-glass telescope we have ample information for grinding and polishing the specula; but, on refractors, the information is scanty. Most probably, had it not been for the persevering efforts of amateurs, the former would not have reached its present honourable position in the astronomical

The tractive friction of the locomotive is between the steel tire of the wheels and the steel (or iron) surface of the 1ails, while the resisting friction that of a lot of well-greased axles in their axleboxes.

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Plumb-Bob.

N.W. OF FRA

[18014.]-IN reply to Mr. Gaudibert's letter respecting the crater (17989, p. 157), it appears to be one which I observed on May 18, 1880, 10 p.m. wrote in my note-book, "There are two crater broken into one another between Fra Mauro and Gambart f, closed to the former on the raised plateau. This plateau does not appear plainly in Mr. Neison's map. The mountain H is plain, and the two craters are south of it. They are each about the size of Gambart f. East of H are three craters, one of which is probably that which Mr. Neison shows at the end of the rill ø. This rill is not visible. The other two craters are not given on this map." The valley Mr. Gaudibert mentions

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I did not notice, but the craters were very plainly visible, and it seems strange he should not have seen them on Jan. 2, 1879. On Oct. 12, 1880, I again saw them, simply making a note that all those seen on May 18 between Fra Mauro and Gambart were visible,

On the former date I made a rough sketch. On the same night I found some craters south of Gambart which are not on Mr. Neison's map. I have sent a note of these to the Selenographical Society. M. A.

JUPITER.

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[18015.]-CONTINUATION of the ephemeris, given on p. 11, of the times when the assumed first meri- Aug. 2 dian of Jupiter will pass the middle of the illuminated disc.

1880.-GREENWICH MEAN TIME.

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Nov. 5 8 31.8 Nov. 24 9 2.6 Dec. 13 19 31-4

Observed Passages.
Prec. End. Middle.
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In this ephemeris the apparent motion of the planet, the equation of light, and the effect of the phase are duly taken into account. The assumed daily rate of rotation is 870-600, which approximately represents the motions of the dark spots of 1834-35, and of 1862. The corresponding period f rotation is 9h. 55m. 27.088. How far the apparent motion of a spot depends on its Jovicentric latitude and on its form and appearance is at present unknown.

The observations of passages of the red spot, which have come to my knowledge since June, are collected in the following list, which may be considered a continuation of the lists published in the Monthly Notices for May and June. The observers and the sources of information are:

lution of the highest test (4. Pellucida), the
of Diatoms may be passed in
whole series
review with only so much manipulation as is neces-
sary to place them, in each case, in the most
favourable position towards the illumination.

A difference of one minute of time in an observed passage produces a difference of about 0-6 in the corresponding longitude, or differences of, say, 3 in the longitudes indicate differences of five minutes in the estimated time of passage. The I am aware that we are here dealing with an discrepancies in the longitudes must suggest to intending observers the necessity of great careful-objective whose resolving-power surpasses that of ness in observing the passages. The sifting of the any other seen in England, save the brought But the same observations in a proper discussion will lead to the over by Prof. E. Abbe, last year. rejection of not a few of the recorded estimates. effect relatively is produced with lenses of less, reIndeed, if the results of superior micrometrical solving power. The lateral motion of the substage, measurements were available, there might be no as above explained, enables us at once to illuneed of employing mere estimates at all. But,minate the object to the exact limit where our obhave such measurements been made, say, in 1878?jectives cease to refract pencils sensibly free from The preceding end of the red spot will be near errors of aberration. the middle line of the disc about an hour before the passage of the first meridian given in the ephemeris, the interval decreasing weekly by about two minutes, so that at the end of December it will be about three-quarters of an hour. 66, Lambeth-road, S.E.

Messrs. Swift seem to have grasped the problem of swinging substages with a determination to test its efficacy in new combinations. A glance at Fig. I shows they have not only applied the lateral motion of the substage similarly to that of the six microscopes above mentioned, but they have added a similar motion at right angles. Each of these movements takes place on a graduated brass sector, OBLIQUE ILLUMINATION BY SWING-so fittel that the achromatic condenser-with POWELL AND

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Mary Ashley, Bath; 4in. Wray refractor; correction of sidereal watch, found by hour-circle of natoreal with estimated uncertainty of less than [18016.]-THE readers of the ENGLISH MECHANIC half a minute; local sidereal times of observed will remember that in the last volume (XXXI.) passages privately communicated. F. E. Barnard, several microscopes were figured, of more or less Nashville, Tennessee; letters 17701, 17816, and elaborate construction:-(1) Messrs. Beck's, (2) 17861 in ENG. MECH. C. E. Burton, Loughlins- the Tolles-Blackham, (3) Grubb's, (4) Ross's, (5) own: 12in. and Sin. reflectors; letter 17955 and Bulloch's Binocular, (6) Bulloch's Monocular-all private letters. F. C. Denuett, Southampton; of which embody a lateral motion of the substage in. Calver; letter 17709. W. F. Denning, in an arc of a circle of which the object under Bristol 10in. Calver; letter 17960. Wentworth examination on the stage forms (approximately) the Erck, Sherrington, Bray; 75in. refractor; letter centre. This lateral motion of the substage enables 17954. G. T. Gwilliam, 35, Lansdowne-crescent, us to utilise with facility illuminating pencils of W. 65in. Calver: private letter. O. Lohse, Potsdam; Astron. Nachr., No. 2325. Mr. Orpin, Loughlinstown; 6in. refractor; v. Mr. Burton's letter. I. V. V., Kensington; 2-2in. Wray, letter 17563. A. S. Williams, Brighton; 5.2in. Calver; letters 17637 and 17833. The times are mean times of Gr., Greenwich; Na., Nashville (5h. 47m. 9s. W. of Gr). The red spot rotates more slowly than be assumed First Meridian, its assumed daily rate, duced from some of the observations of 1879, Eng 870-430, and the period 9h. 55m. 34-06s. The rresponding longitudes" are those deduced fr the ephemeris:-0-17°, the times being recaned in days from October 17.0, 1879.

just that degree of inclination that may best de-
velop delicate microscopical structure; and when
combined with the use of a nearly hemispherical
lens, suitably mounted in the optic axis so that its
plane face is in immersion-contact with the base
of a balsam-mounted slide-say, Möller's Probe-
Platte containing twenty selected Diatoms of
graduated difficulty of resolution-the most difficult
organic test-objects known may be readily ex-
hibited, provided the objective be equal to the task.
For instance, with Powell and Lealand's new 1-12
homogeneous immersion objective, mentioned in
my last (letter 17894, p. 84), the light being ad-
justed to the requisite obliquity for the reso-

which each is supplied--when slid on the sector to the zero point, is exactly in the optic axis of the instrument. Centring-screws, acting at right angles to each sectoral motion (shown in the figure, on the rectangular frames above the illuminating prisms) are provided, so that at whatever inclination the condenser may be placed, a small range of motion can be given to the illuminating pencils, i.e., a slight eccentricity in the radial incidence, which is of importance when we are seeking the highest limits of definition. Messrs. Swift have valiantly adopted a specially small gauge of substage, which is convenient in that they are thus enabled to apply their traversing movements to stands of small size

which I regard as a bold and praiseworthy innovation on our ancient constructions of microscopes. The condensers are fitted with very small bodies and tapering nose-pieces, and the under side of the object-stage is coned out, so that a large range of obliquity is available. The longer sector is carried considerably above the object-stage, and the condenser sliding thereon can be used for illuminating opaque objects; in fact, this condenser may be used above the stage and the other one beneath, both focussed on the object at the same timewhich will doubtless produce curious effects. Both condensers are supplied with a neatly-mounted right angled reflecting prism that can be directed in any position, which is the best reflector known for that purpose. The front sector is so attached, that when not required, it can be swung away, as in Fig. 2.

Not the least ingenious part of Messrs. Swift's alternately the one and the other of the two given is consequently superfluous. In water, with the new design is the convenience of removing the object-points, the amount of the displacement will 4 refractive index each difference of level is whole of this apparatus by a slide attached to the always be considerably less than the actual differ2 3 back of the longer sector; thus the ordinary sub-ence of level. We get, for instance, as focal therefore diminished by a quarter of its real magstage, polarising appliances, &c., can be used in length of a spherical air-bubble in water 98-104 nitude, i.e., reduced to . (Accurate calculation the usual way-if desired. The fact that the new (the radius taken as unity), whilst in reality it gives with an angle of incidence of 18° instead of illuminating apparatus can be applied to most of amounts for central rays to about 1.3. the somewhat smaller value '7395). the stands of moderate size, without interfering with their present usefulness, is of importance to

"This error is due to the fact that the passage of the pencils of light from water into air (the coverFig. 1.

nating this source of error: it is only necessary to "There is, moreover, a simple method of elimireplace the layer of air between the objective and the covering-glass by water: the measurements will then agree exactly with the true values."

I may also refer to a paper by Dr. RoystonPigott, F.R.S., in the Month. Micro. Journal, XVI., p. 294, on "A New Refractometer, &c." Dr. Pigott explains some refined methods of obtaining measurements of the thickness of cover-glass by the difference between the focal lengths obtained in viewing the upper and lower surface,

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the numerous microscopists to whom economy is a

consideration.

I remark a very serious omission in Messrs. Swift's improvements: they have not applied an immersion illuminator.

In reply to query 41854, p. 172. The measurement of the thickness of an object mounted in balsam beneath cover-glass, or that of the coverglass itself (when it cannot be removed from the slide), is a matter of considerable practical difficulty, and unless "E. P." is provided with accurate means he will not obtain satisfactory results. The following quotation from Nägeli and Schwendener's "Das Mikroskop," may be of service to him:"Differences of Level.- Since only those objects can be distinctly seen through the microscope which lie exactly in the plane of adjustment, it is possible to ascertain the distance of two objectpoints in the direction of the optic axis, by measuring the difference of level of the corresponding planes of adjustment with the help of a second microscope placed horizontally, or by a micrometerscrew constructed for that purpose.

"In this latter method there must be taken into account a source of error which influences to a very considerable extent the results of the measurement in cases where the objects are immersed in a liquid or solid medium, for instance, in water. If we determine the tube displacement which is necessary in order to bring into the plane of adjustment I

ing-glass does not here come into account) pro-
duces for the microscopist the same effect as with
the naked eye. That is, a body situated in water
is apparently raised, and, taken absolutely, the
more so, the deeper it lies. The virtual image of
an air-bubble is therefore raised to a greater degree
than its centre, the focal length being consequently
shortened, and for the same reason in general the
vertical distance of the two points is lessened. The
amount of this diminution depends, of course,
upon the angle of incidence of the effective rays,
and is, for small values of it (as long as the arcs
are in the same ratio as the sines), expressed by
-, where n denotes the index of refraction.
Since, then, the incident cones of light are refracted
in the objective as if they consisted of rays of defi-
nite mean inclination, and since this inclination
does not exceed 12 to 18° in the higher powers of
those we have examined, the above expression is
approximately accurate in most cases that occur,
and a trigonometrical determination of the error

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allowance being made for the displacement by refraction. Some idea of the difficulty of arriving at exactness may be formed from the fact that, although Dr. Pigott appears to have faced the problem backed by unlimited mechanical resources, the outcome of his labours was the announcement of his belief that "cover glass is "flint" glass. On the authority of Messrs. Chance, the glass manufacturers, of Birmingham, it was stated in a recent number of the Transactions of the R. M. S. that "cover" glass is crown, and its mean index of refraction is 1.5 to 1.52; therefore Dr. Pigott's "flint" glass was a mistake.

A Fellow of the Royal Microscopical Society.

ECCENTRIC CHUCK-TO "J. L." [18017.]-I THINK "J. L." rather misunderstood my letter, as I did not say I had a new plan of chuck, but that I had a method of turning up the bevels of the ordinary form of English eccentric chuck which made anyone independent of the file or planing machine. About 2 years ago I bought a lathe which I found worth fitting up with chucks for all sort of work, and, thinking I should like to start with an eccentric chuck, inquired the price, and, finding it too costly for me, determined to make one, and in Vol. XXI. of the E. M. found a description of one by "J. L.," and at once set to work, not buying the castings but building it up as I went along. I got on all right till the guide-bars and sliding-plate came to hand, and, being a poor hand with the file, came to a standstill, when it suddenly struck me that if I took a piece of wood 6 or Sin. long and planed it up true to a right-angled triangle whose size would equal the width of the slidingplate, and screwed the base of the triangle to the face-plate, and the sliding-plate or guide-bars to the sides of the triangle, I could, by surfacing outs, turn them up to angles of 45', and in case the wood sliding-plate was turned up on the same side as the was not planed up perfectly true one level of the corresponding guide-bar, and the other edge of sliding-plate on the same side as the other guidebar, both guide-bars being turned up at the same time. The result of this was that, when in place, a very little grinding with oil-stone dust made so perfect a fit that I could pull the slidingplate out so far as to leave only of an inch shake. I find the best way is to mark off the between the guide-bars without any perceptible them on both sides, and turn the bevels last. The bevels and chip them nearly to the mark, then face way I fix the pieces to the wood both for facing and turning the bevels is with glue, as there is r fear then of springing them, as there is by usi screws. A good recipe for the glue is given "Jack of All Trades" in Vol. XXVIII., page under the head "Planing Machine."

Ihe

directions there given are followed, no fear need The lines of force must be isolated as near as can be,
be entertained of the iron parting from the wood; concomitantly, friction-the "diabolus ex machina"
culy let the glue dry. Be sure in using the sliding-will be abolished. The power is pedal, intermittent,
plate to take care that the edge of the level first limited in play, and nominally in a straight line;
turned rests on the face-plate from end to end, or it is required to convert this, firstly into alternate
the two levels will not be parallel. I send sec- rotary motion, and secondly, quoth "Edipus"
onal drawing, which I thing will make everything (41723) into continuous rotary motion. This, to me,
plain.
is gained by cutting the lever-wheel into two, fixing
the halves on two hollow axles, working on the
main axle, separating them adaptively to the legs,
guiding the lever arms straps over the semicircum-
ference of the wheel, and using as nearly as may be
a single line of force, the ratchet-pin. The
following is from rough notes I made some time
ago.

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AA is the face-plate, B the wood, and CC the guide-bars; the dotted line shows the result. I have purposely drawn the edges of the pieces of iron square, to show the effect more plainly. Any angle may be turned by planing up the wood accordingly. I have not tried J. L.'s "plan with an emery disc, as I do those things with a cutter fixed in my drilling and wheel-cutting machine, made from the drawings given in "The Lathe and Forge." Could "J. L." tell me how the motion for the second part of the geometric chuck is got? I can't tell from the drawing given in the "Lathe and its Uses," and no explanation is given in the description. Is any book written giving detailed drawings ?-as I wish to make one.

Thomas Stow.

MEDICAL ELECTRICITY.

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A is the main axle, B one of two ratchets on axle, C loose hollow axle cogged at one end, bearing the fixed half-wheel, containing the ratchet-pin D connecting reversing-wheel, G section of half wheel at C, showing the lengthening of the lever [18018.]-IN reply to Mr. Rankin Kennedy, at the ratchet-pin; E strap or lever-handle. This page 137, letter 17969, it would be of great im- principle of working the tricycle has been most portance to have the machine portable, but not successfully carried out by Mr. Butler, of Wokabsolutely essential. The intermittent currents ingham, with his omnicycle (patented); His would certainly be useful, but are also not essenreverses are pairs of two wheels worked by a tial. What is essential, however, is the power of crossed cord. During motion the peripheral secregulating the current so as to equal the current tion of his lever-wheel can be made to assume of any number of Daniel Muirhead cells, of the sections of circles of three dimensions; this, without size, &c., formerly stated, from two to one hun-altering the pedal stroke, gives in the smaller, dred cells in series. Heating from five to six inches speed, and in the larger, leverage. Mr. Butler of platinum wire would answer for the surgeon's claims the merits of climbing the steepest hills, and machine, and from one to ten minutes the time a road speed of 12 miles an hour. I trust that the requiring it to be so heated, so that the "stiff person replying to "Devoniensis" (41709) will work" of driving would be a minor consideration, have tried the omnicycle and some other maand that being the case, it would be preferable to chines. have it capable of rendering 10in. incandescent. J. M. McCulloch.

TRICYCLES.

Mr. Butler's brake seems second only to Messrs.
Hickling's rack and pinion, both acting on the
tire. My friend Mr. Meteor has had his tire cut
by a piece of flint or glass getting under the spoon.
Marine glue of two kinds, and cements have been
Would not "Moonlight"
tried without success.
(41707) find compressed air better adapted than
steam for the motor power. I predict a glorious
future for the velocipede so worked. A complete
reformation is pending in all vehicular and road

traffic.

Alex. H. Barker.

[18019.]-THAT the lines of the velocipede of the future have not yet been completely laid down is evident in the correspondence and queries on this subject in the pages of the E. M. At present no absolutely reliable information is obtainable, for there is scarcely any one who can claim a thorough practical knowledge of all the best machines, and until some enterprising club takes up the matter and tests carefully the various "makes"-querists IMPROVEMENTS IN THE DETAILS OF who use the superlative degree will see that the usual grain of salt must necessarily season at present their replies. It is not sufficient to know what a man can do with any one tricycle, for, with the same experience, qualities on the average possibly equal may be brought out from another machine."

The changes that have been rung on the positions of the three wheels of sizes equal, and unequal, complicated by safety and patents, are manifold. In some it is questionable whether beauty is a criterion of utility and safety. If to a 50in. bicycle a 36in. axle and another 50in. wheel be added, the larger fork be dispensed with and the back-bone sprung from the side of the axle, we will have a tricycle possessing two standard excellences-two tracks, and clear in front. I proved the advantage of this second quality a few evenings ago; for in turning sharply I was swung out, but alighted on my feet; the machine would have toppled over, but that I retained hold of the stirrup handles of brake and steering wheel. In such handles so applied a third good quality is again apparent, but some of the best machines tacitly ignore one or all of these qualities; are other equivalents obtained? Few tricycles drive both wheels on account of the difficulty of turning; I see no reason why the common clutch should not be applied, to be thrown on and off the key near the hub, by the guide handle, when a turn exceeding 15 to 30 degrees is made.

On looking into the merits of the levers generally used for velocipedes, points of no action are found

BICYCLES.

[18020.]-THE bicycle season is now almost over, and while our machines are standing idle, manufacturers will be hard at work with 1881 designs. My remarks are intended for those who are thinking of getting new machines for next season. Last May, I had a machine specially built for me, which

A

FIG. I

in the lever-arms, and in the inseparable crank; has given such ample satisfaction, that I think T and in the wheels and chain or cogs we miss the cannot do better than give a few of its leading simplicity of the spoke-wheel with power direct. dimensions:

machines; but I do not find the least unsteadiness in consequence. The steering centres are long (5in.), but the handle-bar is placed well forward and low down, so that I have all the advantages of long centres with a very low handle-bar. The backbone is oval, with a very slight taper, and consequently very rigid. The head is open, and, therefore, admits long centres without looking clumsy, as is the case with closed heads. Short centres can be made rigid by having the steering spindle larger than usual. (See Fig. 1.) The top cone should be truncated and much slighter than the bottom, with a small groove (a b) in it. This groove greatly helps the oil to lubricate properly, and by this means a locked steering- a very dangerous thing by the bye-is nearly impossible. The handle-bars, backbones, and hind-wheel forks are, in most machines, capable of very great improvement.

The handle-bar, to begin with, is nearly always too short; it ought not to be less than 24in. Long handle-bars give a great leverage in guiding the machine, and consequently increased steadiness and less fatigue. They should not be more than six inches from the front-wheel tire. The lower they are the greater the command over a machine; but if of handle-bar (Fig. 2) entirely prevents bumptoo low, the legs are apt to bump them. This form ing the legs. With the Editor's permission, I will speak of the backbones and trailing-wheel forks in my next letter, with some critical remarks on the leading machines now before the public.

H. G. S.

FLIGHT AND FLYING. [18021.]-EVERYONE must read with interest any communication from so good a theoretical and practical mechanic as Mr. Wenham, but there are one or two matters that seem to call for remark in what he has stated.

He refers to the impossibility of a screw-steamer exerting any great amount of thrust on the water unless it is allowed to advance, and certainly I can but agree with him; but I remember to have seen a drawing and description of a large dynamometer, erected, I think, at Portsmouth, the object of which was to measure the power exerted by It consisted of a screw and paddle steamers. system of levers whereby a weight was lifted up by the drag on a rope attached to the first lever of the system, the boat remaining, practically speaking, in the same spot, as the movement permitted to the lever was extremely small. According to what Mr. Wenham states, the machine must have been simply a great blunder, and those who designed it and those who erected it equally ignorant of the principles involved in the propulsion of vessels by screw or paddle. Is this Mr. Wenham's opinion?

Again, I would caution would-be experimenters from being tempted to try any flying-machines on the mode of construction shown in Figs. 1 and 2, on p. 133. A friend of mine (now in Australia) was so captivated by Mr. Wenham's schemes that he at considerable expense constructed a large machine on this plan. There was a transversly long, and from front to back narrow, aeroplane, stretched on bamboo framing with an elaborate arrangement of trussing ropes. To this was suspended a light framing containing a rest for the body, and a couple of treadles to actuate a pair of long narrow wings, with an additional arrangement for varying the angle of a short, broad, adjustable tail. The result was complete failure, not the slightest rise could be obtained, and the whole thing was abandoned and destroyed.

Lastly, would not it be better if the members of the Aeronautical Society devoted less of their time to writing papers to prove that flight ought to be practicable, and more to experiments to show that it is? It is now some years, I think, since Mr. Wenham initiated the plan shown in Fig. 5, p. 133, and made partly successful experiments, but

nothing seems to have come of it, or of his latter
device of an undulating surface. Where is the
hitch?
Plumb-Bob.

FLIGHT OF BIRDS. [18022.]-WITH reference to 17962, I have to state that the extract from Mr. Wenham's letter (17776), as given by me (17932), was neither accidentally, nor, to use Mr. Wenham's language, "purposely misquoted," but is there correctly given. I need not, therefore, notice the first two paragraphs of his letter (17962), which appear based on what he wrote a month subsequently, and which is no reply to mine (17932). My criticisms have been directed against what I consider inapplicable or inefficient analysis, and in the absence of conclusive experiment, I stated that the onus probandi lay upon the proposer of a theory that appeared contrary to known dynamical law. When I read of experiments to the point, I am quite willing to acknowledge their importance and bearing. That given by Mr. W. of the oar-blade moved transversely, and which he repeats for my benefit in 17962 in a form which does not prove its efficiency, is contrary to what I should have expected; but though worthy of consideration, it is by no means conclusive, water being a fluid of such different nature to air.

The experiment of Mr. Moy is more to the point, and I am repeating it in a similar form, with results indicative of some retardation; but the experiment really wanted is that of the simple flat plane moving horizontally, such as might be made by loosing such a plane from a height of 16ft. on board a steamer going 15 miles an hour. Mr. W. has stated that at 39 miles an hour, the time of descent is increased eighteenfold. May I ask what proof can be given of this?

We have done with sycamore-seeds; but as Mr. W. again mentions the riddled steamers, I should be glad to know what he considers the dynamical law by virtue of which the water would not enter the bullet-holes. He and " Seveerg" have each given reasons; the latter the more correet, inasmuch as he seems to suppose that the second law of motion applies to some extent, though the side-pressure, willing and ready apparently to act, is not supposed to have time to do 30; an argument against which I have elsewhere given my reasons. But I can come to terms with "Seeverg" if he will allow that there was an indentation around the bullet-holes, which was probably the case. If the holes were as cleanly made as by a drill, and the sides of the steamlaunch, for the sake of argument, of no appreciable thickness, would he not then allow that the water ought to find its way in? I may here remark on the inconclusiveness of indefinite experiments.

I have not had time to read carefully Mr. Wenham's interesting articles on aerial flight, but among other considerations the following have occurred to me. A bird is said to move the weight of a ton of air lft. in a minute, but the weight of half an ounce would move a ton in the same time, and a man could produce the same effect in a second.

66

enormous

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The pressure on each surface of the pelican's
wing amounts to 40,000lb. A condensation of the
air in contact with the wing of 1-4,000th from the
pressure of the stroke, and a consequent rarefaction
above the wing to a similar extent, would be equi-
valent to an upward pressure of 201b.-the weight
of the bird.
M. A.

naked foot, would he call out that it was concase. If a crab were to grip "W. C. E." by the tinuously exerting more than one horse-power in crushing his toes? If a ton weight is suspended with one hand. It was the want of this pectoral from a spiral spring, it may be danced up and down spring that caused the death of De Groof, a few severed from the balloon, the muscular strength of years back, at Cremorne Gardens; for, when his legs on the treadles was not sufficient to keep his wings extended; they became folded together above him, and he was dashed to the earth. Their area when extended was quite sufficient for his safety.

analogy; the term "unyielding" applied to the action on the stratum by extended planes of regular form. Unshapely bodies like wooden blocks, without appreciable force of penetration, can be thrust slowly" into the running stream. A tennis or football may be thrown by an active arm across an extensive lawn; but let the same arın throw the ball out of the carriage-window of a [18023.] M. A.," in 17993, last issue of E. M., train at a 60-mile speed, and the rapidly passing states "that the ice does not break at once is due air will allow it to go but a very little way. to elasticity, not from gravity having had insuffi- main effect of the pectoral muscles in birds giving "W. C. E." then objects to the reference to the cient time to act," and suggests for an illustration support like stretched indiarubber, without musthat the running weight should traverse on a road-cular exertion. Fairly interpreted this is really the ing. I have used exactly the same demonstration way of yielding springs. This perversion is astoundto define the difference between a yielding and non-yielding support. This I explained in the E. M., with illustrations, about five years ago. If a very light kind of rail, or metal band is kept up at intervals of lft. to a stopped level, with a force of 10lb. on each spring, then if a small carriage loaded to 121b. be quickly run over the line, as there is very little weight in either rail or springs, the surplus 21b. will force them down in succession; this will be like the carriage running up hill, or continually surmounting a barrier at a great expenditure of power. Instead of springs, of 101b. each, the carriage running over the line let the rail be kept up by a series of counterweights at the same speed will not then displace these weights, their inertia prevents the now heavy road from giving way, and consequently there is no loss of power from deflection, as in the former case. The ice or plank experiment on water (which to this; we always consider as inelastic) is analogous it is of no case with elasticity. to mystify the If the detached pieces of plank or ice for support were absolutely rigid, the result would be the most definite demonstration for a proof of supporting effect, which would be greater still if the planks were wholly immersed some depth in the fluid. This comparison is quite as applicable to planes in air, and when "M. A.” so coolly states that "the resemblance between the pressure on water-logged baulks of timber (thin planks should have been the right quotation) and that on a surface wholly immersed in elastic air is too slight to warrant the occupation of much space it is difficult to treat such assertions with patience." These questions are not to be settled by mere penand-ink work. Let "M. A." particularise known facts before he ventures positively to contradict the statements of those who base them upon the evidence of real experiments.

use

train at the aforesaid 60 miles per hour, let him Some time, when "W. C. E." again travels in a running hinge edge first, and previously persuading open the door to various degrees (of course, when his fellow-travellers that he has no intention of suicide), and he will gain rather a forcible idea of the action of air against inclines.

The remarks in the concluding paragraph of the letter of "W. C. E.," if written by anyone else, might be considered as extremely personal to himself. It is to be hoped that he is not so far compromised as to be taken at his own word.

F. H. Wenham.

[18024.]-I AM very sorry to see so much space lost in ours by many writing on this question who are unacquainted with the facts of the case.

I lately wrote you regarding it, and to Nature, as the matter is there noticed; and now, as I live in a country where the phenomenon is of daily occurrence, beg to send some data.

Several of our large birds rise by soaring, and to great heights-from 200ft. up to at least 5,000ft. or more, during which ascent they do not flap their wings. Among these are-the pelican, the adjutant and its allies, the vulture, the cyrus (Grus antigoni), &c.

To begin with, they do not soar thus in a calm; 2nd, nor in a long and straight line, or remain stationary.

I have but little to say, as I am loth to venture into Of the following letter by "W. C. E." (17994) a quagmire of mathematical dissertation. This will not help us. We should first achieve something like a practical success in flight, then mathematics may correct or improve our know-flap the wings until some height and impetus is On rising from the ground, they all invariably ledge of theoretical conditions. As it is "W. C. E." gained, and on reaching the (steady N.E. or in his first line professes to "improve" a mere W.S.W.) breeze begin to soar in large circles, or empty formula, taken on rather indefinite data, of rather they begin a huge spiral, that (as far as I the yet unknown force required for bird flight. can see) always trends to leeward, each sweep some "W. C. E." remarks that the up or return stroke 200 yards across or more, and rising 10 to 20ft. of a bird's wing has always appeared to him to be each lap, that passes slowly at, say, 20 yards to leeThe difficulty of birds rising from the ground for performed very quickly. On the contrary, it is ward at every round. In doing this, the wings are flight is noted by Mr. W. Per contra, I would remarkable that the up and down wing-strokes of extended, and the bird-often close and seen with instance the facility with which wild ducks, geese, large birds, in which they can be easily timed, are binoculars-shoots round without any apparent and even swans rise from the water. Sparrows quite isochronous. Pigeons starting off from an effort. The spread of wings may be from 10 to mount quickly from a courtyard, almost perpen-elevation sometimes do just the reverse, giving 12ft.; height of bird standing, 3 to 5ft.; weight. dicularly, to the top of a house or tree. The sky- very quick down-flaps, and at the rise holding 20 to 401b.; and the speed when soaring, from 15 lark ascends, and hovers merrily for an hour or them for an instant nearly stationary: they then to 40 miles per hour. Again, they can be seen two at a time. The hawk uses but little effort, act as aeroplanes. I do not think that there is any soaring away up in the blue, as mere specks, all and that with his long wings, to remain in one loss of power in the upstroke of the wing, as the day long; and with a telescope never seen to flap spot. A chicken, held downwards by the leg, has feathering direction given by the bird prevents it. the wings. little difficulty in rising to a more comfortable I pass on to the second letter of W. C. E." I often come across these big birds in swamps. position. The honey-bee, when hovering over a First, the principle of the soft tallow candle shot and when out shikaring (i.e., after game), and flower, gives no higher note in his buzz-nor, I through a board, has been so well defined near a they invariably rise by strong flapping of the wings, think, so high-as when on his flight; and the century ago, that it needs no alteration in the and if it is a calm, this is kept up, except for short martin and swallow readily rise from the ground. terms of interpretation. Second, "W. C. E." de- intervals at times, or while alighting again. If Birds with long wings are not intended to rise fremurs at the term, unyielding stratum, or support there is a breeze above, they may begin to soar at quently from the ground, and one reason of their obtained by the wings of a bird in rapid motion. 100ft. up or so, and to circle round and round, difficulty would seem to be that they have not The term unyielding" is meant only so far that sufficiently near to hear the loud "sing" of the muscular power to raise themselves by the extre- the bird does not descend. This quotation was feather tips, and to note the whole bird closely and mity of their pinions, which naturally first come from a paper, intended more to convey a popular casily, as it quite passively swings round and round to the ground. Their leverage-power is calculated, idea than a mere scientific description, as waves are in the huge circles. When first rising and flapping. so to speak, for only half that distance, viz., to the said to run "mountains high." Strictly speaking, the long neck is generally extended in front, and centre of pressure on their wings. They have not few supports can be termed solid; foundations on when soaring, is doubled on the back, so that the on the ground room for the full sweep of their clay yield, and upright tombstones in old church- point of the beak only projects beyond the breast. wings. By the way, I should say that their large yards sink at last to the forgotten departed be-Resting thus, as it were, they gradually ascend to breast-muscles are a certain indication of the power neath. But passing from grave to gay, I cannot immense heights, quite 8,000ft., and 1 fully believe they possess and use. letter of "W. C. E." He says that he was once forbear a laugh at the concluding portion of the 10,000ft. travelling in a railway-carriage at the rate of 60 miles per hour, and put his head out against the

To turn to another subject. Mr. W., I think, says that an aerial screw machine, revolving in a vertical position, does not ascend quickly; and (a), if inclined, mounts quicker, but (6) would meet with much atmospheric resistance in the direction of its course. Why is this as to (a), and especially as to (b)?

The mass and inertia of the air are very small, and the twenty or more ounces displaced by the fedican are incommensurate with the twenty pounds to be supported on rising. No notice has been taken of the pressure and elasticity of the atmosphere and the alternations of rarefaction and condensation "unyielding" stratum of air, but his head did not attendant in a greater or less degree upon every yield, like the "wood and candle," and "his skull diatorbance. was not fractured." I may say that there is no

All the birds before noted do this. The pelican, when "fiying." flaps and floats as per diagram, only seen, though, when it is travelling.

&c., floating for long distances, and
The phenomena of sea-birds, as the albatross
which

I have seen, is the minor problem included

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