66

or

feet of water, by which the tubular valve is always kept full and "plump." This little supply tube is so placed as to be passed by the coulter, &c., and to permit the valve to be lifted up and pressed back again into its seat. g is the travelling pi ton-head; h, the rib frame of the travelling gear; h, the balance-weight; l, m, n, o, the hollow grooved rollers, made like ordinary sheaves," which gradually lift the tubular valve out of its jaw-shaped seat, to permit the coulter to pass with the piston; the first and last of these, and o, are narrow enough to pass up between the jaws, or into the longitu tudinal slot, and are of hardened steel; is the roller, with a slightly concave edge or rim, which, attached to the porch of the leading carriage s, presses down the tubular valve into its seat, something like forcing a continuous cork into the neck of a bottle, and so leaves the main ready for fresh exhaustion after the passage of a train; t is the coulter of plate iron five-eighths of an inch thick, carries the rollers, piston, &c., and attached to the perch s.

THE

GREAT BRITAIN'S" LAST

PERFORMANCES

VOYAGE

-THE
OF HER EN-
GINES COMPARED WITH THOSE OF THE
"GREAT WESTERN."

Sir, I have read your account of the third voyage of the Great Britain in last week's magazine, and am both surprised and annoyed at the results there detailed. I learn from the account referred to that the consumption of fuel is 63 tons per day, which, if employed as it ought to be, should give out fully 900 horses power without any extraordinary degree of expansion; but we are told the steam was generally cut off at 13 inches of the stroke, so that it must have expanded between five and six times; hence the power realized should have been very much above 900 horses. Mr. Guppy (the Director of the company, under whose superintendence these engines were made) stated, at the Institution of Civil Engineers, that the steam was generally cut off at one-sixth of the stroke, (of 6 feet;) hence the statement, here referred to, of its being cut off at 13 inches, is no doubt correct. Now, to me, the unaccountable part of the affair is this: The boilers of the Great Western supplied sufficient steam at

3 lbs. for 15 strokes, without working the expansion gear; and I know from indicator cards I have seen, that when so working, the cylinders of 72 inches diameter were filled six feet out of the seven with steam of 3 lbs. above the atmosphere. Now, at 15 strokes, the two engines of the Great Western would consume 10,177 cubic feet of steam per minute. The heating surface of the boilers of the Great Britain is about 8,500 feet, that of the Great Western was about 4,000 feet. The quantity of coal consumed in the latter was about 24 cwt. per hour, the former 53 cwt. in the same time; hence it follows that the quantity of steam generated in the Great Britain should be more than double that of the Great Western.

But if we take the diameter of each of the four cylinders of the Great Britain at 88 inches and the revolutions at 14, we find that the quantity of steam that should pass through the engines in one minute (cutting it off at 13 inches of the stroke) is 5,096 cubic feet. Now, I should like to know what goes with the rest; for that a much greater quantity is gener ated I have no doubt, although not suf ficient to produce the most satisfactory results, because the surface in the boilers should have been at least 12,000 feet.

I should advise Captain Hoskins to have his engines carefully examined by a practical mechanical engineer of known standing, conversant with the application of steam to marine engines. He will have found by this time that it is perfectly useless for Directors to become their own engineers, or the constructors of railways to attempt marine engine building.

I am, Sir, your obedient servant,

London, Nov. 3, 1815.

COMMANDER

G. S.

HOSEASON'S LETTERS ON WORKING STEAM EXPANSIVELY IN THE ROYAL NAVY.

Sir, I have just read your reprint of Mr. Hoseason's letters to Sir W. Parker, c, with Binnacle's eulogium, and your commendation thereon.

Although I admire talent in any grade, and have always looked favourably on the efforts of the Otways, Hoseasons, and others, for the benefit and enlightenment of their benighted navy brethren in the

and resistance to the water's motion, arises from its action against the containing sides, it follows, that a greater velocity, upon the same inclination, will be generated in the latter section than the former; that is, it will run more water through the same section, or the same quantity of water being determined, since a lesser inclination will suffice to generate the same velocity, the canal will empty itself at the upper end to a lower level, and thereby better drain the land.

"To resume my former illustration, of a canal 10 miles long, and 10 feet deep of water, being supplied at one end, with as much water as will form a cascade at the other end, running a quarter of an inch deep; the width is not material, if we suppose the cascade to be of equal width with the canal ; though, for the sake of fixing our ideas, we will suppose the width 60 feet, I say, that (without applying to calculation) I should not expect the rise of the canal at the entering end, in order to discharge such a quantity, to be above 2 inches higher than the cascade end, whereas, was you to conduct the same quantity of water for 10 miles, upon a bottom inclined from the solid top of the cascade, so as to run nowhere deeper than a quarter of an inch, I do not suppose an inclination of 100 feet would bring it, though you was to bring it on boards planed and jointed as smooth as art could make them.

"I therefore consider a canal having 4 feet natural descent in 16 miles, dug upon a dead level bottom, as equivalent to a canal dug, in the whole, 2 feet deeper than if the bottom is made to rise 4 feet in that length; and with less expense of leakage; because a canal dug with a sloping bottom, in the whole 2 feet deeper, must for one-half of it be dug under level of the natural discharge; and the level bottom will have this further advantage in dry times, when the quantity entering becomes very little, that the surface of the canal at the upper end will fall nearly to the level of the lower, whereas in the inclined bottom it can never run below the bottom; this circumstance may sometimes indeed be a disadvantage, by reducing the water in the drains, in the interior part of the drainage, too low; but where descent is scarce, and consequently the drainage is liable to be languidly, or imperfectly performed, it will be a very great help, in getting the drainage of the same more early completed; and after all there are always means of restraining it."

Letter from Mr. Grundy to Mr. Smeaton, July 31st, 1770.

"I cannot but be satisfied with the reasoning contained in your solution of that

question, as being metaphysically true; yet I cannot say I am convinced, that the highest advantages obtainable in point of velocity, by digging a drain to a dead level, can by any means compensate for the additional expense which must attend such practice, especially where the fall is anything considerable; and was that to become a fixed principle, it might be construed to extend and operate in some cases, so as to introduce 10 or 20 feet unnecessary depth to be dug; and as it but rarely happens that the falls we find in countries to be drained are by nature made sufficient to convey their waters to sea, by proper drains made on a parallel to the inclination of the plane of the surface, I yet think it more eligible to pursue that practice; and more particularly so, because in the other instances, where the fall is found inadequate for a natural drainage, engines must be introduced to perform it artificially. I do not recollect that we proposed making our drains on a dead level in our schemes for the drainage of the country connected with the Foss Dyke."*

Mr. GILES contended, in spite of what had been stated, that a depth of two feet below low-water mark at sea was sufficient for drainage, as all marshes were situated at a level between high and low-water marks.

Sir JOHN RENNIE said he could not have any hesitation in admitting the correctness of the principle of level bottoms, for main drains, in very flat districts; but almost every case of drainage had peculiar features, and in the Ancholme, it must be remembered, that in a distance of about 14 miles there was a fall of nearly 20 feet; while in the catch-water drains, into which the high-land waters descended with a certain velocity, there was a far greater fall; therefore the rising bottom was not only not injurious but was highly advantageous in that instance; because it enabled the floods to be carried away with sufficient rapidity, without incurring the extra expense of cutting the drain so much deeper. The case of the middle level was totally different there, as Mr. Walker very properly stated, the fall being only about 18 inches in 30 miles, there was no alternative but to make the bottom of the drain level, and to give great sectional area, because those drains must act as reservoirs to receive the great mass of water which inevitably fell into them, and which could only be discharged during the short time the sea sluices were open at each tide; but even presuming that the cill of the outfall sluice be laid from 6 feet to 8 feet below low-water mark in the Ouse; still when the

* Vide Smeaton's Reports, vol. i. p. 82 (4to. 1762.)

river Ouse should be improved below Lynn, as was contemplated, the low-water mark in front of the sluice, proposed by Mr. Walker, would be lowered 5 feet or 6 feet, so that the cill of the sluice would then be only about 2 feet below low-water mark, as Mr. Giles had stated.

Sir GEORGE CAYLEY observed, that he had been for more than thirty years one of the directors, for carrying out the provisions of the Muston Drainage Act, including about 10,000 acres of land near Scarborough. This drainage was effected under the direction of the late Mr. William Chapman, of Newcastle-upon-Tyne, who had great experience in such matters. The drains appeared to combine in their just and most economical proportions, the two adverse principles at issue in the previously expressed opinions. In that extensive and gently rising marsh, the dead level principle was adopted from the lowest outfall, till the surface of the water in the drain, at ordinary times, was within about 4 feet of that of the soil, which level was found sufficient for the purpose of draining the adjacent lands. From this point, the drains took the average rise of the marsh, and continued it for several miles; thus furnishing, at the cheapest possible rate, a very useful and efficient drainage, to all the lands under the Act. Had the dead level been continued throughout the whole length, the expense would have been enormous, without rendering the drainage more complete, and had the dead level not been brought up to the point named, many hundred acres of the lower portion of the swamp would not have received any benefit.

The general plan of the Muston Drainage might be thus stated. The small rivers Hartford and Derwent, with several brooks, held their courses through an extensive marsh, and in times of heavy rain they overflowed their banks and flooded the land to a great extent. No expense whatever was incurred for cutting channels, deep enough to convey away the flood waters of these rivers, or brooks, but they were allowed to keep their ancient levels, and embankments were made near them, on each side, by cutting deep back drains, for carrying the dead water from the lands, and casting up the soil excavated from them, on to the sides next the rivers or brooks. By this process, all the great body of water was conveyed, in times of flood, within these embankments to the lowest outfall; and the deep cutting, which he considered the sine qua non of an efficient drainage, and the expensive part of it, was entirely confined to such moderate-sized drains as were sufficient

811

merely to convey the dead water from the land. Another practical advantage, of the deep back drains being contiguous to the embankments, was, that when they received any injury from cracking, after long droughts, or the burrowing of moles and water rats, and thus permitted the flood-water to pass in some degree through them, the back drains interrupted it, and preserved the land from injury.

The original cost of this drainage was about 40,000l.; and the annual repairs averaged about 8007. The improved rent was obtained at about four or five years' purchase.

The expense of this drainage had been much increased by local circumstances, and which could scarcely be supposed to occur in any other cases, and therefore it was unnecessary to detail them; but these circumstances took place at a distance from the marsh land, and in no way invalidated the state of the case.

Sir JOHN RENNIE said, that Mr. Telford, in his drainage works, had as nearly as possible acted upon an uniform system, similar to that which had been described; but that, in particular cases, it was necessary to adopt peculiar methods. It was certain, however, that in all cases it was essential to commence the drainage at the lowest point of outfall, and to work inwards, towards the head of the marsh. In the Bedford Level that system had been neglected, and to that circumstance Sir John Rennie attributed much of the difficulty that had been experienced.

It was always a point of importance to restore the rivers to their natural state, of main drains for the country. At Boston, in the year 1826, he recollected seeing the bed of the river nearly dry, at the time of wha ought to have been high water. Since thent the outfall below Boston had been improved, by making a cut across Burton's Marsh and improving the channel of the river upwards, upon a plan proposed by him; the effect of these works had been such, that vessels, drawing upwards of 14 feet of water, now arrived at Boston.

THE MIXTURES MOST

SUITABLE FOR OBTAINING BY ELECTRIC PRECIPITATION COPPERPLATES OF SUPERIOR FINENESS. BY PROFESSOR VON KOBELL.

[From Translation in Silliman's Journal.] "In order to attain a uniformity in the current, I tried what would be the effect of adding to the solution of the sulphate of copper other salts, which resist the action of such feeble currents as are here called into