finger to the bottom of one of the rafters, although the rollers are only lin. in diameter. by 5-16in., riveted to each other back to back, and of the best kind of polished mahogany or teak, to the reverse frames throughout the vessel. with two sets of compasses (the compasses to be Mr. Berthon's instructions are as follows:- BALLAST.-The space between the floors to be provided by the owner), but to be both carefully "Into a flat piece of ground, say, a grass-plat, filled in with cast-iron, securely bedded in adjusted by the builder, who is to have the vessel drive about 30 good stumps (mine were 3in. Portland cement, a channel leading to the wells swung for the purpose, and compensated to square), in a circle, using a central stump and ra- to be formed in the cement, and the cement to be counteract the attraction of the hull of the vessel; dius-bar for the purpose. Then, when the stumps finished off with a smooth surface. Deck stringer binnacle to be fitted with two good-oil lamps and stand 8 or 9in. from the ground, saw them down 2 or water-way plate of iron plate, 13 4-16in. reflectors. BOATS.-One four-oared mahogany 3in. (better, not quite 2, so as to get at the top sur-wide amidships, tapering to 9in. by fin. forward life-boat gig, 22ft. long, supplied with necessary face with a plane before the ring is taken away), at 5ft. from the central peg, and nail to each stump a horizonal arm to project about 6in. beyond the outside of circle. These horizontal pieces should be all in one level. Now have ready a quantity of strips of good deal (I used swamp-elm, as stated above), 2in. wide and in thick. Tie (nail) one end of the first strip to one of the stumps, bend it round as far as it will go, tie (nail) again, and "butt" another to it until the first circle is complete. Now bend another set over that and nail and clench as you go. Add another and another, bending and nailing till you have a ring consisting of about 7 (circles of) strips, which will be about 44in. wide. Use French nails." The use of French or wire nails is important to avoid splitting, and also for another reason perfectly understandable by any one who has ever opened a packing-case fastened with them. Also it is well to drive in a lot of the wire nails, about 3in. long, both from the inside and the outside of the ring, so as to tie all well together. G. H. Evans. and aft, supporting a covering-board or plank sheer oars, boat-hook, gun-metal crutches, rudder, KEEL to be of sound American rock elm 8in. | to be laid tapering fore and aft following the run sided, and not less than 4in. deep, to taper fore and of the vessel, to be side pinned with galvanised aft to stem and stern post. A solid iron false keel, dowells; seams to be properly caulked and payed not less than 6in. deep, to be cast and securely with marine glue, and to be scraped and planed; fitted underneath the main keel. The builders to ends of planks to butt lin. into covering-board. make the moulds and provide the iron. ROUGH TREE TIMBERS, or bulwark stanchions, of best-seasoned heart of oak, mortised into the plank shear 3 in. moulded by 4in. sided, at deck; the bulwark skirting to be of teak. MAIN RAIL, of American ash or Canadian elm, 6in. by 3in., fitted with square brass sockets, and 15in. strong galvanised iron stanchions all round; stanchions fitted with strong galvanised twisted wire life-line, to go round the yacht. STEM.-To be a sound piece of seasoned English oak, with a grown crook 8 in. sided and moulded as shown in drawing, properly butted and scarfed into keel, secured with yellow metal bolts as required by Lloyd's rules. Stem to be finished off with cutwater, and figurehead, to be selected. Convex gunmetal stemband and shackle for bobstay, The stern-post to be of English heart of oak, sided and moulded as required; of extra size in way of the shaft. The after part to be secured with a gun-metal shoe. Knight-heads and apron to be of seasoned English oak. Dead-woods.-The upper dead-woods to be of English oak, and of sufficient depth to receive the iron keel-plate, and to be bolted with galvanised-iron bolts. Frames to be spaced 18in. apart from moulding-edge to moulding-edge, according to Lloyd's requisitions, and to be of the best quality of angle-iron, 24in. by 24in. by 5-16in. Reverse frames of best angle-iron, 2in. by 2in. by in., to be riveted to every frame and floor-plate, extending across the middle line to 14in. up the bilges in the way of the engines and boilers, and under the ballast. FLOOR-PLATES to be 18in. by in. deep, and 1-16in. thicker under the engines and boiler. The floor-plates to extend on each side, as required by Lloyd's rules. Three watertight bulkheads, made of best iron plate, all in. thick, and stiffened in an efficient manner, with angle-iron stays, as required by Lloyd's rules, the bulkheads at either end of engine-room to extend up to iron deck beams, to be securely riveted thereto, and to be fitted with sliding watertight iron door. MIDDLE LINE KELSON, of double angle-iron, 24in. by 2in. from cistern in engine-room, and japanned copper receiver, 12 brass cloak-pegs, and 12 strong brass clothes-hooks. An enamelled copper bath, to be fitted with waste-pipe leading to bilge under floor of each cabin, with sea-cocks, provided with brass lever-handles, and lead pipes communicating with condenser in engine-room, fitted to supply hot water to baths, and also to washstand, with suitable taps for hot and cold water-supply, all pipes to be fitted with union joints. DECKHOUSE 9ft. long by 6ft. wide, over the companion, provided with two glazed doors, the whole built of teak, and glazed on three sides with best in. plate-glass, 4 panes framed and sliding in grooves, to open like carriage windows; deckhouse to be fitted with seats on three sides, with lockers under, with a small, but strong, hinged table in centre and with an American elm grating over the deck. STEWARD'S PANTRY to be fitted according to plan, with dresser, shelves, liquorholes, racks, hooks, drawers, lockers, and to be provided with all requisite fittings; iron bed-bunk, to fold up; brass pump, and pipe communicating with the tanks; galvanised iron tanks under pantry floor; eight-gallon wicker-covered filter, with nickel-plated tap; door from pantry to saloon to be divided in the centre horizontally. Fore-cabin to be fitted up on one side of the vessel, forward (and opposite to the pantry), similar in all RUDDER.-The main portion of rudder to be of one respects to the two sleeping-berths aft (but withsolid piece of English oak, and to have a carefully-out bath), and to be fitted with a 6in. side-light fitted watertight rudder-trunk; to have strong copper braces, fitted with a goose-neck head, to work on a centre with a hoop and socket on the deck in a teak chock; spare mahogany tiller, with brass fittings; pintles of gunmetal. Steering gear to consist of an approved steering apparatus, a tiller aft, and wheel of best Spanish mahogany, with polished brass nave and polished brass hoops, all handsomely fitted and French polished. Binnacle and compass COAMINGS AND FRAMES OF SKYLIGHTS.-All of best-seasoned teak, oiled, and varnished with two coats of copal varnish; skylights to be all thoroughly weather-tight, and provided with sufficient brass fastenings below. Bulwarks, of lin. Malabar teak, varnished inside with two coats of copal varnish, and fitted with a sufficient number of 14in. scuppers, hinged with brass hinges; height of bulwarks from deck, 2ft. 4in.; an opening port or gangway on either side. Side lights in cabins, pantry, and saloon, of best in. plate-glass, 6in. in diameter fitted in brass flanges, and with stout hand-screws; to be hinged to open inwards, and fitted with stout indiarubber washers; frames of side-lights in saloon to be nickel-plated. with brass flange, handscrew, indiarubber washer, &c. WATER-CLOSET of Downton's best make, to be fitted with panelled door, brass lock, bolts, &c., at foot of companion, opposite the above sleepingcabin. MATE'S AND ENGINEER'S CABIN, to be fitted abaft the engine-room, with sleeping-berths, nest of drawers, lockers, washstand of clean pitch or red pine, and basins, hinged oak table, shelves, racks for charts, &c. Cabin and men's quarters THE SAND DRIVING-CLOCK. [18232.]-SINCE the subject of cheap driving purposes. aft to be lined with pitch or red pine, varnished, signal-flags, and mahogany signal- locker, densing engines, guaranteed to indicate 85 h.p. the latter to be provided with hinged mess-fitted where required, and with 38 par- Specifications for the same, and drawings of table, benches with lockers under, and four titions. BELL.-Strong brass yoke, with brass engines to be supplied by the contractor, and to hinged iron bunks, hooks for four hammocks fittings, for carrying a 15in. bell, to be fixed where be approved by the surveyor on behalf of the and bedding, washstand and lamp, and w.c. aft. required (bell to be provided by owner); fog-horn, purchaser, and to be guaranteed to propel the W.C.'s, two in number, one at foot of companion speaking-trumpet ; electric bells and speaking- vessel in smooth water at a speed of not less than forward and one aft, both of Downton's best make, tubes in deckhouse and salcon, communicating 8 knots per hour. The funnel to be double, and with latest improvements. Sail-room, with 30in. with engine-room, pantry, and mate's cabin. to be made to lower telescopically. Boiler power circular teak hatchway, on teak coamings, secured GRATINGS.-Stout painted brass rods and brass to be excessive. Vanderdecken. with brass lock on the aft side and brass hasp and guard-wires, fitted over all skylights; grating of padlock on the fore side; shelves in sail-room, American elm fitted so as to form a seat on lined with pine varnished, to be fitted abaft the taffrail over counter, and grating athwart the deck men's quarters, and to extend into the counter of in front of the seat-gratings to be fitted to bows the vessel. GALLEY.-A suitable cooking apparatus, and platforms of both boats; grating of American of the best make, to be supplied as selected, and to elm at foot of companion and on the floor of deck-clocks for telescope has been under discussion, and be fitted forward in forecastle with copper funnel house, as above specified. TANKS. Galvanised particularly the sand-clock, a brief description of and tubing of sufficient length, and hood on deck, water-tanks (lined with Crease's patent compo- 6in. refractor may be of some interest. At that one I devised some seven or eight years ago for my and with all requisite gear for cooking, and with sition) fitted under saloon, pantry, after-cabin, time I had never heard of the use of a column of patent ventilation coamings; shelves and dresser &c., and provided with the requisite pumps, gear, sand for this purpose, but subsequently learnt that to be provided as required. The floor under- &c., for filling, and brass pumps for emptying the Mr. Bird, of Birmingham, had made use of the neath the stove, and all the woodwork near same; tanks to be connected and to contain same principle, which appears to be described in the stove to be neatly covered with sheet-lead, together not less than 250 gallons. Pipes for nailed with copper tacks. A strong table, with filling the tanks to be fitted on deck, with brass the letter of Dr. Anthony (p. 257). I found none of coaming and hinged flap to drop. Locker for half deck plates; suction pipes and pumps from tanks though, of course, the performance of any clock of the objections mentioned by this correspondent, a ton of coal under platform, separate locker, par- to galley and pantry. MEAT-SAFE.-To be fitted on titioned off, for the chain cables; both lockers to be deck, lined with zinc, and fitted with louvre boards this description would necessarily be inferior to a lined with stout hard wood. Storeroom to be for ventilation at each end (safe to be provided by first-class movement, regulated by a pendulum. fitted up between the saloons and the after-cabins, owner); to be scraped and polished by the a great many experiments and trials with reference Before reaching the form finally adopted, I made and at the foot of the companion, and with all re- builders. PAINTING.-The whole iron framework to quisite shelves (with coamings), drawers, racks, have two coats of best Torbay and Dart Company's bottom and its regulation, method of collecting with to the size of the sand-tube, the aperture at the lockers, brass hooks, brass locks, &c. SALOON red oxide paint, inside and out, in addition to the the polar axle, weight to be used, &c. This conENTRANCE OR COMPANION to be fitted with teak stairs, priming of the same paint. Outside of the vessel trivance has long since been done away with, and making one half-turn, treads fitted with roughened to have two coats of paint, each well rubbed I am obliged to give the dimensions of the several brass, and brass nosings, walnut wood hand-rail down, and to be finished besides with two coats of parts from memory, so they may not be rigidly on each side, French-polished movable American best black varnish, and with a 2in. gold line along elm or ash grating at foot of companion. Light the moulding the whole length of the vessel, gilding exact, but will be near enough for all practical iron ladder fitted to forecastle, and also to the to be varnished. Bottom up to water-line to have engine-room, and sliding hatch with hood over all, two coats of Jesty's patent composition. Carved of teak, fitted over forecastle hatchway; hatch work at stem and stern to be gilt, and varnished fitted with brass slides and proper fastenings and work bright-varnished, as required. Mate's, bolts. Teak ladder with teak hatch, hatchways, engineer's berths, and berths for crew to be bright coamings, &c., giving access to the after-cabin. varnished. COOPERAGE.-Six oak water breakers MASTS AND RIGGING.-Two; of best red mottled or of (6 gallons each) with galvanised hoops, bungs and Oregon pine fixed on deck in strong cast-iron gal- funnel, 6 canvas wash-deck buckets, 2 oak harness vanised sockets properly secured below, each mast casks with galvanised hoops, 2 mess-kids of teak to be fitted with a brass ratchet winch and with with brass hoops, and 2 bread barges, 12 enamelled brass spider hoops and brass belaying pins, and the iron plates, eight enamelled iron cups and saucers two topmasts to be provided each with gilt metal for the crew. FENDERS.-Six large cork fenders trucks, each fitted with two gunmetal sheaves, and cylindrical and painted, 2ft. 6in. long, four 12in. each truck fitted with two sets of signal halliards, pudding fenders covered with white canvas, 6 lifebowsprit, tepmasts, boom, and jibboom of best sea- buoys, 4 best hemp mats, and four 14in. cocoa-nut soned and mottled red pine; yard for square sail. fenders. PIG BALLAST.-A sufficient quantity of Two gaffs, both of best red pine, and all other re-iron pig ballast, in 28 or 561b. pigs, to be supplied quisite spars and gear, to be of the best quality, and carefully stowed, to bring the vessel down to Galvanised wire standing rigging, shrouds, and the desired load-line, each pig to have two coats of stays. Stays to each mast fitted with runners and red lead before being put on board. SUNDRIES TO tackle, running rigging, all of the best Russian BE SUPPLIED.-A log line and reel, Walker's last and Manilla hemp. All blocks patent, fitted with patented log for recording speed, hand-line and gunmetal sheaves, and internal galvanised strops. lead, four deck scrubbers, 2 paint rubbers, 2 mops, Galvanised and leathered horses for mainsail and 1 squeegee. Finally, the general design of the fore-staysail, and a strong gun-metal goose-neck for vessel to accord in all respects with the drawings the main boom. SAILS AND AWNING.-One suit of as finally approved and settled. The whole of the sails of best material and quality, and made by materials and workmanship to be of the very best Lapthorn, to be provided, and one suit of storm- quality, the whole of the standing and running sails, painted sail-covers, and painted bag for each rigging, blocks, tackles, runners to be fitted comsail and each portion of the awning, the name of plete in every detail. The peak and throat each sail painted on its respective bag. Painted halliards to be of the best Russian hemp, and the canvas covers for each hatchway, and for deck-jib halliards of galvanised chain. Skylights in house, and skylight deck awning in three portions, with vandyke side curtains 9in. deep, and fitted with crows-feet, stretchers, &c., galvanised iron stanchions fitting into main rail to support awning, ANCHORS AND CABLES.-Two bower anchors, 1 Martin's, 1 Porter's or Trotman's, as may be selected, and all as may be required by Lloyd's rules for a vessel of 100 tons; 1 cwt. stud chain cable of best make, and four shackles, two swivels, chain cable, 120 fathoms in length, all galvanised, and, together with the anchors tested and proved at a public machine, so as to secure the letters A and CP, in Lloyd's Register; sufficient number of shackles and swivels. cabin and saloon to be glazed without putty or side and aft side of each mast with The sand-tube was about 2in. in diameter, of drawn brass, smooth inside, and of uniform diameter, inside and out. It extended from a point six or eight inches below the intersection of the declination and polar axles to the ground, making the total length some seven or eight feet. It was fastened to the north side of the pier, which, in this instance, was a stick of timber twelve inches square, set firmly in the ground. The motion of the falling weight was communicated to the telescope through a tangent screw, twelve threads to the inch, engaging with a sector attached to the polar axle in the usual way-by a clamp and screw. The radius of this sector was some seven out interfering with the main casting of the stand. or eight inches, as long as it could be made withOn this tangent-screw, which was about 6-10ths of an inch in diameter, was placed an accuratelyturned wooden drum about 2in. in diameter. One end of a very small soft steel wire, not much larger than a thread, was attached to this drum, being wonnd around a sufficient number of times to allow the driving weight, which was attached to the other end of the wire, to fall to the bottom of the sand-tube. A small crank was attached to the drum and tangent-screw, by means of which the weight could be quickly wound up to the top of the sand-tube, when it was necessary to pour the sand back into the tube. The sand-tube could not be conveniently placed directly under the drum of the tangent-screw, and the wire was, therefore, passed over an intermediate pulley, placed, of course, directly over the upper end of the sand-tube. The axle of this threads being equal to the thickness of the wire. pulley was a screw, the distance between the This made the wire always wrap smoothly around the drum, and prevented any crossing of the coils which would, of course, change the rate of the clocks at once. I do not remember the weight used, but it was of lead, some four or five inches long, and a little smaller than the inside diameter of the sand-tube. the tangent-screw, would never require changing, It will be seen that the sector, with reference to the elevation of the weight to the top of the PATENT RATCHET WINDLASS of best make and column restoring the sector to the original position. sufficient power, provided with fire-engine action This sector was kept pressed against the threads of and handles to unship. Bowsprit bits of heart a tangent-screw by a weight of a few pounds atof oak stepped and properly secured to strong tached by a cord to the sector, and hanging down iron plate, riveted securely to the beams with on the east side of the pier. Hence, the weight on an iron stanchion to run down to the kelson the sand had but little more to do than turn the directly underneath the same: SIDE LAMPS.tangent-screw, the telescope itself offering but One set of strong and best made copper side lamps the lower end of the sand-tube, and regulating the little resistance. A good many ways of arranging with proper screens and galvanised irons, and in accordance with the Board of Trade rules; one flow, were tried before a satisfactory plan was masthead and one copper "riding" light all diopfound. At first, the diameter of the aperture was tric and with parabolic reflectors; a rack to be varied by a fine screw, the hole being approxifitted in forecastle for lamps; two copper deck- teak cross-pieces, and all other necessary belaying mately round within the limits of the use of the lanthorns, with globular glasses; one ditto for A stout galvanised in. iron tube, for a adjusting-screw. This was not satisfactory. forecastle, one for crew aft, and one for engine-fitted with a flange to the outside of, and flushing funnel, made of very thin brass, not thicker sounding-apparatus, will be required to be securely The form finally adopted was a gradually taper room. STANCHIONS. One pair of strong brass stanchions, 3ft. long, with a pair of white cotton with, the surface of the garboard strake, the tube than a sheet of ordinary paper. This was brought sideropes and white pudding fenders; accommoda- rising perpendicularly into one of the cabins. The nearly to a point, and the size of the aperture tion or side ladder of teak, with brass fittings, vessel to be further provided with two pairs of polished, and made to ship on either side of the enlarged by spreading the opening with a slightlyof 5in' stream hawser best waterlaid and tarred it was easily remedied by a little flattening of the vessel, and to be varnished. ROPES.-60 fathoms tapering piece of steel to the required diameter. If, by accident, this opening was made too large, hemp; 60 fathoms of 3in. ditto; 60 fathoms of opening, and then again enlarging, if necessary, 2in. warp ditto; one coil, 25 fathoms, of lin. rope with the tapering steel. This was found to be for throw-lines. FLAGS.-One for each masthead, ENGINES.-The vessel to be propelled by a pair much more satisfactory than any screw or lever 1 ensign, and one complete set of (19) commercial of compound high and low-pressure, surface-con-adjustment, and really much less trouble to adjust. bitts. properly-constructed galvanised or oak legs, and When the proper aperture was once found, which could be done in half an hour's time, it required no further attention during the time the clock was in use. Various plans for giving slow motion in Right Ascension were tried, among others that of having an extra hole, considerably larger than the other, for the sand to flow through when required to bring the star up to the wire or to the centre of the field, and shutting off both when the star was behind, both apertures being worked by cords from the eyepiece. Finally, the extra flow of the sand was discarded, and the opening and closing of the regular aperture effected by a single string from the eyepiece end of the telescope, the pulling of which alternately opened and closed the end of the funnel. The sector was clamped lightly to the polar axle, so that the instrument could be moved to any part of the heavens while the clock was working, or in a condition to work, by pulling the string. In fact, the clock was always ready to carry the instrument whenever wanted, by the observer simply pulling a thread or very small string attached to the eye-end of the telescope, and stopped in the same way. The difference in moving the telescope when the sector was clamped sufficiently to allow the clock to pick up the instrument instantly, and when not clamped at all, was hardly appreciable. In setting on a star it was placed in the preceding side of the field, and brought to the middle by arresting the flow of the sand for a moment, the next pull of the same string starting the sand instantly, and retaining the star at any point in the field desired. This arrangement I found in practice to be all that could be desired. Indeed, this kind of driving-clock was more convenient to use, so far as its manipulation was concerned, than any other I have seen. The sand at the bottom ran into a tin vessel, holding perhaps four or five quarts, the opening for the reception of the stream of sand being about the size of the neck of an ordinary bottle. am convinced the sand-clock is much superior in 6ft. 7in. bogie-engines, which are to run the heavy I wish to call attention to two errors of the S. W. Burnham. FLUID DRIVING-CLOCKS. Dec. 16. ATTRACTION. C. N. G. [18236.]-THE theory which "Aletheus" has advocated (letter 18213) is not new to me. Many years ago, I came to the same idea, but found just as many inexplicable conditions as there are in Newton's theory, or even more. For instance, it would require a shell round the universe in order to keep it together, while attraction produces centres, i.e., planets, which have, at least to a certain extent, freedom of movement. The position of the axes of the planets cannot be explained by it no more than by Newton's theory. A body in a vacuum ought not to be drawn towards the earth, because the pressure is equal all round. A body moving in the direction where there is the least pressure in nature, then that part would require that there should exist a partial vacuum; and how would that be created without a pull? I might mention many other conditions; but will take it as a favour if "Aletheus" will throw some light on the above question. The arguments that a young man experiences the least pressure in the direction of his sweetheart or, I might say, towards a joint of roast beef, I cannot accept; but It was perhaps a minute's work to wind the weight up to the top of the sand-tube and pour the sand back. My impression now is that the clock would run with the tube filled with sand about an hour and a quarter, or an hour and a half. It was not often that it required to be replaced more than twice during an evening. I should state that a much larger sand-tube was first tried and found not to work well. Different methods of connecting the falling weight with the telescope were experimented with; among others a thin piece of steel, a THE CONTINUOUS BRAKE QUESTION they are not worth contradicting, to my thinking. watch-spring, being attached to the sector, the other end passing around and fastened to a small axle. The weight was attached to a large wheel on the same axle, the motion of the falling weight being thereby reduced to the proper amount for the movement of the sector. These various devices, after trial, were abandoned, and I have every reason to believe that the plan finally adopted is the best that can be devised. The sand used was the commonest kind of lake IN FRANCE. 1879 a most interesting series of experiments was The results obtained matic and Smith's vacuum, each fitted to a train At the conclusion of these experiments it was or river sand, the only preparation being to sift it of sand will work the best. It is evident that some, if not all, of the objectionable features of the sand-clock described by Dr. Anthony were due to the improper construction and arrangement of the mechanism. For instance, connecting the eye-end of the telescope with the weight would seem to be, in practice, about the worst arrangement that could be thought of, taking the varying positions of the instrument into account, and the apparent necessity of a readjustment of the sand-flow for objects in different portions of the heavens. Such a plan is no more fit to be used with a sand driving-clock than with a spring-governor, or any other driving-clock, and certainly would not give satisfactory results in practice. For a cheap home-made contrivance I with this brake, and as a first instalment the year. I say this without disrespect to "Aletheus." THE REESE FUSING DISC. [18237.]-CAN any of your readers inform me disc has been confirmed in England, or is it an if the marvellous account of the Reese fusing American story only? Here, in Manchester, it has been tried in one conditions named in your and other journals, of the largest machine establishments, with all the without a sign of success. Not a single atom would move, or a molecule dissolve. Yet I find in the MECHANIC of Nov. 5th, p. 208, that Mr. Watson, of Lincoln-road, Peterborough, in a long letter, actually ventures on an explanation of the absolute against any rash speculator in solutions of phenomenon; but in order to make his solution inexplicable matters, he says:-"I claim all forces, thermic, electric, and magnetic, which may be developed by the motion of contiguous surfaces." Can anybody explain this ambiguity of forces, or give any reliable description of this magical disc? Thos. Almgill. 41, Great Cheetham-street, Broughton, Manchester, Dec. 18. Will some chemical or BENZOLINE GAS-AND ALBO-CARBON that the company have already 177 engines and I learn from the Western Railway of France ARRANGEMENT. 1,460 carriages fitted and working with the [18238.]-ALL those who have experimented Westinghouse brake at the present time; and also with benzoline (air) gas will know that, as far as that the directors have lately given a large order illumination is concerned, the result is nil. When for the fitting of 375 engines and 2,500 carriages. I read the favourable testimony given by "J. P." The French engineers are justly proud of the of the albo-carbon arrangement on p. 626 of last way in which the continuous brake question is vol., and in other parts, I thought it would just be receiving attention upon their railways, and they the thing for increasing the illuminating power of express the opinion that "their trains will be fitted benzoline gas. I nave given it a fair trial, and it with brakes fulfilling the conditions of the Minister is an absolute failure. of Public Works before our English trains are other reader kindly point out the reason of this? provided with those required by the Board of For the benefit of those who are working with this the question is now settled in Scotland. Trade." They must, however, remember that gas, I may say that for producing a continuous stream of air for making the gas, the arrangement figured on p. 358 of Vol. XXVI. is absolute perfection, and far superior to the air-pump and blowing business recommended by Mr. W. G. Chambers in the last volume. If any reader' of this knows of any burner which will produce a fair light with this gas, I shall be glad to hear of it. I am aware it can be done by using gasoline; but as I have been trying for the last five or six years to procure this in reasonably small quantities, at a fair price, without avail, I have given up all hope in that direction. Leicester. Clement E. Stretton. NEW EXPRESS ENGINES UPON THE [18235.]-EVERYONE who is interested in loco- ! 402 ENGLISH MECHANIC AND WORLD OF SCIENCE: No. 823. I should be glad to hear the experience of any As a generator of the gas, the form described Felliscliffe, via Leeds, Dec. 20. ELEMENTARY MATHEMATICAL OP- In order to ascertain the refractive index of such a medium as crown or flint-glass, it is generally ground into a prismatic form; the angles of incidence and emergence then admit of easy measurement. Let A B C be the section of a prism whose refracting angle is BAC; D a radiant point from which a ray D E is incident on the surface A B at E. By the first refraction it is turned into the path EF, and after the second refraction it emerges in the direction F G. Join DG, produce G F to M and DE to intersect it in I; draw the normals HEKN and L F K intersecting in K. Let the angle of incidence DEH = I, angle of refraction FEK = R, angle EF K, and the angle of emergence or second refraction GFI = r. To an eye placed at G the object D will be seen in the direction G F M, and the angle D I M, contained between the incident and emergent rays, is called the angle of deviation, since it is the measure of the deflection which the ray undergoes by its passage through the prism. This angle admits of a minimum, that is, it has a certain value which is less than any other; and it can be shown that this occurs when the angles of incidence and emergence are equal. The following useful relations may be deduced = from Fig. 2. Let the reflecting angle BAC the angles at D and G = 0 and respectively, and the angle of deviation. A = =A, 8 = (0 + $). and equating the values (2) and (3) we have r = (2); (R+ i)....(1); (I + - A). (9++ A I).... (4). The value of i cannot (3) be obtained in a simple form; it is -, i = 2 1(tan. 4. tan. A 2 2 r+ I .cot. + 1) 2 1).. 1 DEC. 31, 1880. prism that the angles of incidence and emergence equivalents in terms of the sine, we obtain sin. I is evident that if we know the refracting angle, can easily find 8, and thence the refractive index. and also the angle of incidence or emergence, we DG and DE are parallel, then the angle at G is the minimum deviation, for it is obviously equal to the angle DIM. I quote from Coddington an Now, if the point D be so distant that the rays example showing the application of this method. prism with a refracting angle of 21° 12'; it produces a minimum deviation of 24° 46'. Required A fragment of silicate of lead is formed into a the refractive index: prism, the refracting angle must not exceed a and GH IJ the drawing-board. EG and DH, certain value. Let the ray DE parallel with the FI and F J are diagonal braces or ties, proceeding In order that rays may be transmitted through a K being the position of the usual adjusting plates, face A B, then the refracted angle FEK = the from convenient places on the legs to near the feet of the stand, forming an equilateral triangle, But A a + i, and the condition of emergence is that ia strut which, by means of a right and left hand critical angle of the medium, which we will a. should not exceed a. Therefore, if A is equal to screw, is capable of being expanded. (R+i), (eqn. 1), or in this case A = corners of the board. F D and FE are ties from the leg C K to the other legs respectively. DE is BEH diminishes, FEK diminishes and E F K or greater than 2a, i is equal to or greater than a, increases; therefore, if the refracting angle be too and the ray cannot emerge. great to allow a ray parallel to BE. to emerge, But as the angle much more will it be so for any ray incident at a lesser angle DEH. Hence, in order that a ray may pass through a prism, its refracting angle must be less than twice the critical angle of the medium. glass 66° 6'; hence, light cannot pass through a is caused by internal reflection. It has four plane M H NOTE.-To prove that the deviation is a minimum = tan. (5). Now, if we know r and i, we can easily determine the index of refraction (u), for sin. r Bin. From the above equations we can find the value of the index from any position of the radiant point; but several angles have to be correctly measured, and the equations are complicated, so that on the whole the above method is more theoretical than practical. But let us so adjust the together with D E, form four "struts," and the being a prismatic compass stand, the ties of crino- The The use I make of it is for a camera-lucida stand, in place of the heavy, awkward, walking-stick stand usually supplied with the instrument. Of course, it is applicable to many other uses. ties may be made of light-pitched chain, and so the adjustment may be altered to give obliquity to the board. The crinoline steel has lightness and rigidity to recommend it, and with it the stability and firmness of the stand are very remarkable. The drawing is to a half-inch isometrical scale, the distance between the feet being 4ft., and the height from L to K 3ft. 8in.; the drawing-board 24in. by 16in. Robt. J. Lecky. ASTIGMATISM. astigmatism recommended by "H. A. W.❞ in letter 18150, p. 303, is, perhaps, well suited to display to [18241.]-THE diagram for the investigation of himself the defects of his own vision, but it is not to be recommended as a generally useful test for the defect. determining the presence or absence of astigmatism is to be found in Pray's letters, or the well-known The most convenient apparatus for the purpose of diagram consisting of lines radiating from a point. A glance at the former is sufficient to inform any amount of the astigmatism is no doubt a more diffione, whether, for practical purposes, he is the subject of the defect or not; but to ascertain the cult matter. We shall suppose for the sake of illustration that the indistinctness of vision for lines in a vertical but that the horizontal lines constituting the testdirection is corrected by the use of a concave lens, object are clearly visible at the ordinary distance. If, now, by further experiments we find that a concave lens of, say, 20in. focus renders the vertical lines distinct, we have only to purchase from any optician a lens concave to 20in. in the vertical direction, and plane in the horizontal, that is a cylindrical concave lens of 20in. focal lengths. Again, if "H. A. W." requires a concave lens of 20in. focus to see even the vertical lines distinctly, and he is obliged to use a lens of 10in. focus to render the horizontal lines clear, he must, of course, get a lens concave in the vertical direction to 30in., and in the horizontal to 10in. The optician will not attempt to produce this complex curvature of the lens on one of its surfaces, but will give him a lens concave in the ordinary sense on one side and ground on the other side, so as to be concave only in one direction; in other words, a lens spherical on one side, and cylindrical on the other. To judge by "H. A. W.'s" account of the defect from which he himself suffers, this would probably be the proper description of lens to correct his vision scientifically. It should of coure be accurately placed in the spectacle-frames, so that its centre should correspond with the pupil, and it should be otherwise most carefully adjusted with reference to the eye. Remembering this, we cannot surely be surprised that "H. A. W." spent two hours in his researches in the Birmingham lens-grinding establishment before he was thoroughly satisfied with a pair of spectacles. It seems to me that two months would have allowed him a better chance of meeting with a really suitable lens or pair of lenses, and I would respectfully suggest to him that an interview with an ordinary oculist would even now be a prudent measure for him to adopt. Opticians bear somewhat the same relation to oculists that chemists hold with reference to physcians. A sensible patient does not expect to have an unusual or obscure malady recognised across a counter, and remedied off-hand by a nostrum from the tradesman's store. If "H. A. W." called at one of the eye infirmaries in London, he would no doubt have obtained more reliable information than he seems to have acquired. It is quite possible that he suffers from a much more complex and less easily corrected variety of astigmatism than that to which allusion has been made. And he seems to be altogether unaware that short-sighted people such as he are frequently the subjects of a motor derangement of the eyes, for the relief of which a lens is requisite, which is neither concave, convex, nor cylindrical, but which is prismatic. Pathologist. MOTOR FOR SMALL BOATS. is, that it would not work with certainty, and in Now let me see what can be said in its favour. N M FIG.I IF A .D. FIC.2 suspended, to hold the substance required to be [18242.]-"PANTAGRAPH's" request for criticism other like the divisions in a telescope. Near the on his design for a novel motor for boats is end of the outer tube there is a pan (N, Fig. 2) one that I feel much inclined to respond to, and I trust to be able to do so, as I would desire, in a fair, impartial, and kindly spirit. Allow me to tell him that the idea is both novel and ingenious, and deserves a more favourable criticism than, unfortunately, I am able to give it, as there are several fatal defects. My daily experience with gas-motors has, I can assure him, demonstrated to me many matters of fact that had not dawned on me before. But in the matter of friendly examination, I must ask him to consider the following matters and conditions that are inseparable from his idea as it now stands. (1) He would require to charge the reservoir C either with an auxiliary pump, or by working the arrangement by a lever up and down several times to less than its full stroke. The requisite pressure having been obtained by giving the main and airpump, pistons a slight extra travel, it would | (Fig. 2). open the valve E, and the compressed inflammable When using a weighing-machine thus conair would force down the main piston by its ex-structed all tangible weights are, as we have said, pansion. Then the piston, being thus driven dispensed with. In the ordinary lever rod a given down, passes the platinum fuse. The explosion force or power, P, would be applied at the end of occurs, and then we have two other sources of the long arm, in order to counterpoise a weight, failure. (2) The force of the explosion would lift W, at the end of the short arm. Instead of this, the valve É, and fire the compressed mixture in C, advantage is taken in the tubular balance of the with the result that very possibly the force of the weight of the moveable tube itself, the centre of explosion thereiu would tear it into pieces like a mass of which forms at once its weight and its bomb-shell. Or (3) if this was prevented by some point of suspension (Do, Fig. 1). The shifting of modification of the valve, seeing that the curve of this point to or from the fulcrum is analogous, explosion falls so rapidly, how are we to make therefore, to altering the length of an ordinary certain that the air-pump piston would descend low enough to fully open the exhaust valve? Especially as after the explosoin both pistons would be moving against the resistance of the pump-spring, and also the weight of the column of water under the main piston. If the exhaust be not fully open to the very end of the stroke, the next charge of gas would be so vitiated that it would fail to explode. Then, again, you propose to have a valve on the air-pump's piston. This would most certainly fail in action to do what was required of it. The question of requiring a battery is a minor point against it, but that might be got over another way. Thus far I have been obliged, in fairness, to certainly condemn the idea. I am quite sure, as it The two tubes, one within the other, are now used as a single lever rod. A point is found on this rod, from which, if suspended, the system will balance itself. This point is the fulcrum F lever rod, and applying a tangible constant weight an ordinary lever-rod, at the same distance from the fulcrum F. To graduate the balance, draw out the tube a short distance until the system is horizontal. At the point where it emerges from the outer tube, let a mark <o be made for zero (Fig. 2). Now put a weight, say, of forty grains, into the pan, and push the movable tube out until equilibrium is re-established. The length of tube thus withdrawn is measured from <o to q, Fig. 4. If this FIC. 4 ID' interval is divided into four equal parts, it is obvious that each part will indicate ten grains; and if each of these is subdivided into ten parts or intervals, each will indicate a single grain. When marks are made on the movable tube to show these several indications, the graduation is completed, and the scale is fit for practical use. The principle involved in the construction of this balance is the same as that in the ordinary lever-rod-viz., that the power remaining the same, there is an equality in proportion of the weights in the scale to the graduation on the movable tube. That this is so is proved by direct experiment and verified by calculation. The novelty of this balance consists, therefore, not in the principle involved, which is as old as the lever itself, but in the interesting fact that a couple of tubes may be so adjusted as to render them available for developing this principle in all its integrity, so that minute quantities may be weighed with certainty, and with greater facility than with any other ordinary balance. Theory of the Tubular Balance. 'We call Ao, Fig. 1, the centre of mass of the part of the outer tube which lies to the right of F, and Bo the corresponding point on the shorter part of the same tube which lies to the left of F. Co is the point of suspension of the scale, and Do the centre of mass of the entire inner tube, which acts as movable weight. The scale being empty, the movable tube is pushed out until the beam becomes horizontal; the point where the movable tube emerges from the outer tube is the zero point o, from which the graduation commences. The forces in operation may now be considered. Let L M N P, Fig. 2, be the weights which represent the forces acting respectively at the points Ao Bo Co Do, namely, L and M are the weights of the outer tube on either side of the fulcrum F; N is the weight of the scale and of all that belongs to it, while P is the total weight of the movable or inner tube. The moments of these forces are the amount of the force multiplied by the distance of its application from the fulcrum F. Let A B C D, Fig. 3 stand for Ao Bo Co Do; then, 1st, L x FA, P x FD are the moments which tend to rotate the beam from left to right: and 2nd, M x FB, N x FC are the moments which tend to rotate the beam from right to left; we have, therefore, the equation (1) L x FA + P × FD = M x FB + N x FC. Let now a known weight of Q units of weight, say 40 grains, be placed in the pan, and let the inner tube be drawn out until the beam becomes horizontal. It will be seen that, while the points A B C have not changed their position, the point D has shifted its place from D to D1. A mark < is made on the movable tube at its point of emergence from the outer tube, indicating the weight now balanced. In this state of the balance the weights applied at A and B remain unchanged. The moment of the weight of the scale has becomePX FD1 and as F D1 = FD + DD1 and moreover DD1 = 09 we have P x FD+Px eq, the moment of the movable tube. (2) LXFA+PFD = The equation of moments thus becomes(MxFB+NxFC +P × oq +QxFC Now the terms of the upper line in this equation and the terms in equation (1) are respectively equal to one another. We have, therefore(3) P x oq = QX FC. If, instead of a weight Q, we had used a weight Q1, the movable tube would have required to be withdrawn from o to another point 71, making the equation (4) PX 091 = Q x FC where the weight P and the length FC have not changed. Are og and 0q1 proportional to Q and Q? That they are so is manifest, because the equations (4) and (3) being divided member to member, give |