MR. R. MALLETT'S NEW LONG VALVE AND MAIN FOR ATMOSPHERIC RAILWAYS.(SEE CURRENT VOLUME, MECH. MAG. P. 232.) THE objects in view are to diminish the cost of the main and valve, simplify their parts, and diminish leakage which occurs to so great an extent with Clegg's valve. The main is cast with a pair of jaws, one on either side of the long slot, through which the coulter travels. These jaws are formed to a particular curve, (see fig. 1,) and are cast against "a chill" by which they are obtained perfectly smooth, fair, straight and hard, and thus the cost of "planing" the valve faces is avoided. The valve consists of a continuous-hollow tube or hose, of woven hemp, coated throughout with caoutchouc, like the tube of a stomach pump, or other such instrument. This tube is maintained full of water or brine in cold climates, and when it is closed as a valve, is forced in between the jaws of the main, and acts like a sort of continuous cork. As the coulter, &c., travels along, the tube is lifted up a few inches out from the jaws, by suitably formed rollers, and as soon as the coulter has passed, it is pressed back again into the cavity between the jaws by a roller pressing upon its upper surface. In place of a hollow hose full of fluid under a constant small head, or of compressed air, a compound continuous cork formed of four cotton ropes embedded in caoutchouc. This is, in fact, one of Brockedon's patent stoppers of indefinite length. Either arrangement would admit of sufficient extensibility in length to allow the lifting up and pressing down of the valve at the passage of the coulter without injury. The outer surface of the valve, in either case, should be coated with an unguent, which will not act on the caoutchouc; if vulcanized India rubber be used, com. mon palm oil will answer. Pinkus's valve was a continuous flat band of leather, and failed, because when close, it had no tendency to keep in its seat, and its edges were thrown up by the pressure of the atmosphere on its centre part. Hallette's valve consists of two continuous tubes full of compressed air, by the elasticity of which they are forced against each other, and the main thus attempted to be made staunch; but the serious defect appears to be, that the tendency of the atmospheric pressure upon the outside of these artificial lips is to force them asunder, so that the exhaustion of the tube tends to produce, in place of to diminish, the leakage of the valve. The present contrivance, which has something in common with both Pinkus's and Hallette's arrangements, through invented long before the latter published his plan, appears free from the disadvantages of either, and to possess several advantages not offered by any other valve proposed. a a The letters refer in common to all the figures. Fig. 1 is a transverse section of the improved main and valve. is the main; bb, the valve seat, the opposite faces chilled; c, the tubular valve in its seat; when raised at the passage of the coulter it assumes its cylindrical form, as shown in dotted lines, d d d d, passing over the sheaves, or rollers m, &c.; t is the coulter seen endwise; h, the rib of the travelling pis ton. Fig. 2 is a plan and section horizontally of the atmospheric main, a a; bb, the valve seat or jaws, cast with "chilled" faces-(these are best seen in section, fig. 1.) The lengths of main are put together with abutting rabbeted flange, or rather lugged joints, at every 15 feet, with a flange of India rubber inch thick between, the elasticity of which allows for expansion of the main, and yet keeps the joint air-tight. Fig. 3 is a horizontal section of the tube, and plan of the piston. Fig. 4 is a vertical section of the tube, and elevation of the piston. Fig. 4a is an elevation of the entrance of the tube. Fig. 5, transverse section of the valve as raised; fig. 6, a transverse section of it as closed; fig. 7, a transverse section on the line A B of fig. 4; and fig. 8, a transverse section on the line C D of fig. 4. From the facility given for support of "the cone," by the "chill," for casting, the valve seat faces on the main, as thus designed, can be as readily cast in 15 feet lengths as in 9 feet, which has been the limit of Samuda's practice. c is the tubular valve of woven hose, covered with caoutchouc, or of caoutchouc and cotton solid: it is here shown hollow, and is maintained full of water by a small flexible tube d, at either end of the section of main joined to the extremity of the 66 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 or frame of the travelling gear; k, 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; r is the roller, with a slightly concave edge or rim, which, attached to the porch of the leading carriages, 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. 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 comunder whose superintendence these pany, 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. WORKING STEAM EXPANSIVELY IN THE Sir, I have just read your reprint of Mr. Hoseason's letters to Sir W. Parker, with Binnacle's eulogium, and your commendation thereon. c 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 THE PRINCIPLES AND PRACTICE OF DRAINAGE. science of the steam engine, still it is going rather too far to allow them to appropriate to themselves as their individual inventions, the most common laws and fundamental rules of civil engineering, as applied to marine purposes. The work published by Lieut. Otway is so full of glaring errors in almost every page that it would be necessary for the naval student to unlearn almost every thing he had there imbibed, before he could enter on a sound course of instruċtion. Let us now advert to the letter of Lieutenant (now Commander) Hoseason to Sir W. Parker, republished in No. 1158. We there find him accounting for, and explaining the statement of Captain Oliver, "that in the Phonix he could, full consumption, steam only 9 knots, and half consumption, 7 knots," in this sort of a way, that "the resistance of fluids is as the squares of the velocities,' consequently 72 49 and 92 = 81, or nearly double! Surely, comment is unnecessary on so absurd a figment as this. = If he had told his brethren, that "the required horse power is as the cubes of the velocities," he would merely have disseminated a fact that has been known to marine engineers for many years past, almost before steam was used in the navy, and subsequent practice has so proved this rule that no doubt exists of its truth. We elucidate it by the statement of Capt. Oliver. The Phonix (at the time referred to) was fitted with Maudslay's engines. Cy linders 55 inches diameter, and 5 feet stroke; therefore collective power = 110 horses. The maximum velocity is stated to have been =9 knots, and with half consumption =7 knots. The cube root of 100 is=4•79* The cube root of 55 is = 3.80 Then, as 4.79 : 9 knots :: 3.80 to 7·18 knots, or just the asserted speed at half the consumption of coals, simply because half the power was exerted, or only 55 horses; but all this had nothing to do with the question of the expansion of high steam in its true sense, because the Phoenix had then no expansion valve, and the object was obtained by the throttling of the valve,” 66 309 Mr. Hoseason writes as if the theory of expansion was a new thing; in fact, that he had almost discovered its practical application: this is a fallacy that he will do well to forget; it was known to engineers and is to be found laid down by competent authors many years before his time; nay, it had been practically used in private steamers. If his friends claim for him the mere reiteration of known principles, upon the dull comprehensions of Lords of the Admiralty, he has undoubted merit-although in this Otway certainly took the lead. I mean when he was in command of the Echo, fitted with Cornish boilers, working under a pressure of 15 lbs., and expanding about two-thirds of the stroke. (See his book published by Sherwood, 1834.) Within the last three years, it was a rule of the Government engineers "that no boiler should work under a greater pressure than 5 lbs. per square inch.' What then are engineers to do with such absurd regulations as this pressing upon their exertions? It is well known that in private vessels a much higher pressure is used with a great degree of expansion, and of course, corresponding economy in the consumption of fuel. These remarks are not written with any ill feeling towards Lieut. H., but I think he claims far too much. I am your obedient servant, THE PRINCIPLES AND PRACTICE OF [Concluded from page 331.] "From the above premises it follows, that where a given quantity is to pass off, the greater the section is, in which it passes, the less inclination or difference of level will suffice, or on a given difference of level, the greater the section the more will pass. "The section may be increased by widening, but if it runs shallow, suppose 1 foot deep, on 20 feet wide, if the width is increased to double, it will undoubtedly, on the same inclination, run a double quantity, viz., it will act as two drains of 20 feet (or nearly so) but if you dig the 20-foot canal I foot deeper, so that the area of the section of its water may also be 40 square feet; in the latter case the whole body of water will be contained in a circumference of 24 feet, whereas in the former it will be surrounded by 42 feet; and as a great deal of impediment 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.) |