"When the circular valve is placed on the seat, there is stagnant atmospheric air within the aperture. On the condensed air being admitted into the pipe, the stagnant air is put into motion, and before it can overcome the inertia of the valve, is forced between the outer parts of the valve and its seat. The air, while being thus forced is, however, compelled to diverge from a circle, whose diameter is 24ths to one of a larger diameter, and is consequently dilated and attenuated. The impulse given by the compressed air on its first admission to the stagnant air in the pipe, causes the stagnant air to commence the process, but the compressed air follows instantaneously, and through the force with which it is impelled by the original moving power, is projected under the valve, and there forced to diverge with a velocity proportioned to the amount of the projectile force. "The projectile force acting through the stream of compressed air, and the peculiarly shaped and confined space through which the air is driven, are then the causes of its dilatation, until its degree of rarity is beyond that of the atmosphere, when atmospheric pressure on the upper side of the valve preponderates. "This view will, perhaps, be illustrated, by supposing the compressed air at the edge of the aperture to be an elastic ring of 24ths diameter, and that every part of this ring shall be struck with equal force from the centre, in a radiating direction to the circumference. By the time that the ring is projected to a sufficient distance to be a diameter of, say 4 inches, it will be stretched from a smaller to a larger circumference, and every part of the ring will be equally stretched or attenuated. It is not however necessary that the substance projected should be elastic, for if the ring were made of lead, the effect would be the same; or if grains of sand, or small lead shot, could, in like manner, be thrown from a centre, in all directions around, it is clear that as they were removed farther from the centre, the grains or shot would be more distant from each other, or the stream of them would be more attenuated. "It has been suggested, that the formation of the vacuum may be accounted for from the known tendency of a compressed spring, when liberated, to fly beyond the point at which it will finally settle. But this action of a spring is only one instance of the operation of a general law of nature which is applicable to all bodies. When any body elastic or non-elastic is put in motion, its inertia causes it to continue in motion in the direction in which it has been impelled until its force is expended. The force of a liberated metallic spring is expended in the effort to overcome the tenacity of the substance of which it is composed, while the force of a cannon ball, fired into an earthen bank, is expended on the resistance presented by the earth; but it is projectile force that is expended in both instances. Addenda. "In a short time after the phenomenon of the adherence of the air-valve was observed by Mr. Roberts, he ascertained, by experiment, without knowing that it had been done before, that water, when forced through a conical pipe, with considerable velocity, will draw out other water, placed below in an open vessel, if one end of a small tube is inserted in the conical pipe, and the other end is immersed in the water, in the vessel below: thus showing that water, an inelastic fluid, produced the same effect that air did, when rushing out in a stream, confined in a peculiar manner. And at the time this paper was going to press, water was, by pressure from a column of considerable height, made to issue from a pipe with a valve placed over it, when the valve, instead of being forced off by the issuing stream of water, was found to adhere to the seat, at a small distance from And when the apparatus was inverted, and the valve consequently placed below the seat, upon the water being permitted to flow, the valve, instead of obeying the law of gravity and falling by its own weight, or of being driven off by the force of the stream of water, adhered, with considerable firmness, to the seat." it. APPENDICES TO MR. WEALE'S NEW EDITION OF TREDGOLD ON THE STEAM- Mr. Weale has recently published two very useful Appendices to his excellent edition of Tredgold on the Steam-engine and Steam Navigation. The first, (called Appendix C,) is by that industrious and promising young engineer, Mr. Samuel Clegg, jun., whose "Practical Treatise on Coal Gas" we had occasion some time ago to commend to the favour of our readers; and is occupied entirely with the well-known Gorgon engines, as fitted to H. M. S. Cyclops. "For a correct delineation of our subject we are indebted," says Mr. Clegg, "to the liberality of the Messrs. Seaward; the late Mr. Samuel Seaward, whose premature death we have to deplore, having expressed his anxiety for a thorough investigation of the Gorgon engines, and which it is now hoped will be duly appreciated and valued by the profession." It will naturally be inferred from this, that "a thorough investigation" is what is contained, or at least attempted, in this Appendix. Strange to say, it contains nothing of the kind! On the contrary, Mr. Clegg is at pains to assure his readers, beforehand, that "no attempt will be made to solve questions which admit of discussion, or have been contradicted, or differently answered by different authors-no theories will be given-no examinations of merit will be entered into; but merely simple explanations given of the engines as they are the work they actually perform—not what they might be made to do." (Page 3.) "Thorough investigation," therefore, there is absolutely none-nothing of the sort, to be "duly appreciated and valued by the profession." The utility by no means small-of Mr. Clegg's Appendixconsists solely in its giving an exact and minute account of "the engines as they are," and "the work they actually perform." We are disposed, however, to blame Mr. Clegg less for not trying his hand at the " thorough investigation," than for sundry not very wise reasons which he gives for refraining from the attempt. "It is not opinions which are wanted now, for they are already various." "Much has been said about the Gorgon engines-much both for and against-ingenious arguments have been used on both sides, but the end finds us exactly as we were at the beginning.'' "In every description of steam-engine, and in all machinery whatsoever, there is some fault; no class of work is perfect, and it would be unreasonable to expect the absence of every defect in the Gorgon engine." All this, and much more to the same effect, we feel bound in friendly sincerity to tell Mr. Clegg, is sad twaddle. Words-nothing but words. We should be sorry to think that he is unequal to the task of such a "thorough investigation ;" but we must at the same time confess, that we never met with reasons for not grappling with a subject, which savoured more of inability to do so. Where, after all, exists the necessity for "the thorough investigation" talked of? We cannot help thinking that Mr. Clegg must have greatly misunderstood "the anxiety" which the late lamented Mr. S. Seaward "expressed" on this point. Mr. Samuel Seaward was not ignorant, of course, of the excellent explanatory pamphlet on the Gorgon engines, written by his brother, Mr. John Seaward; and the utmost he could have meant to convey to Mr. Clegg must have been, a wish to have the arguments of that pamphlet weighed, sifted, and tested, in every possible way. The plates illustrative of the construction of the Gorgon engines are ten in number, and produced in a style which does unexceptionable credit both to the author and publisher. Some valuable assistance in this department, from Mr. E. J. Biven, late draughtsman to Messrs. Seaward and Co., is very gratefully and properly acknowledged. They are all "given to a working scale, or with figured dimensions." From the letter-press explanation we must find room for a few extracts: The Slides. "The slides, which admit the steam above and below the piston, and open passages therefrom to the condenser, are those patented by Mr. S. Seaward in 1835, and are peculiar in their construction, quite independent of one another, and therefore capable of separate adjustment. Any degree of expansion can be given to the steam by altering the throw of the steam slide by the grooved lever and horizontal rod: the slides themselves are of cast iron, inches thick, 11 inches deep, and 25 inches wide, faced against a cast-iron packing piece, dovetailed into the nozzle, and jointed with elastic packing. The slides are kept up against the packing pieces simply by the pressure of the steam. "The exhaustion slides are kept tight by the effects of the partial vacuum behind them, and work against a packing piece on the side -nearest to the condenser. "The steam slides are attached to their rods by knuckle joints, which suffer them to open outwards, should they be acted upon by undue pressure inside the cylinder, thus forming safety-valves. "The adjustment of the slide is such, that when the piston has descended 4ths of its path the steam is shut off, and when quite at its bottom stroke, the lower slide-valve is open ths of an inch, and the upper exhaustion slide open 2 inches. = "The area of the steam passages is 3.25 x 21 68.25 square inches, being in the ratio of 1 to 47-13 of the area of the cylinder. = "The area of the exhaustion passages is 5.25 x 21 110-25 square inches, being a ratio of 1 to 29-17 of the area of the cylinder." The Saline Detectors. "The Saline Detector applied to the present boilers consists of a glass tube fastened on to the front of the boilers, and open to them below the water level, containing, therefore, water of the same density. Two glass hydrometer balls are placed within it, one heavier than the other, but the weight of both being greater than their bulk of seawater, when containing of salt, they remain at the bottom of the tube in the first instance. When the evaporation and saturation has continued until the water contains of salt, the lighter ball rises to the surface, and it becomes necessary to blow out. The second ball is adjusted to rise when the saturation is equal to about, which is beyond the limits prescribed by practice. The "blow-out" pipe is carried to within a few inches of the bottom of the boilers, so that the more dense water is ejected; and when the lighter hydrometer ball sinks again, the blow-out cock is shut. "The safety-valve loaded to 34 lbs. per square inch insures the boiler against an explosion; but the engine-man requires a gauge to enable him to keep up his steam to the proper elasticity, and this instrument consists simply of an inverted syphon-tube of wrought iron, partly filled with mercury, open to the steam at one end and to the atmosphere at the other. The pressure of the steam, therefore, depresses the mercury in one, and causes it to rise an equal height in the other leg: a rod of wood, floating on the surface of the mercury in the latter, marks lbs. upon a scale of inches. The bore of the tube must be equal in both legs, otherwise the mercury will not rise so much in one as it sinks in the other, and, as a consequence, will not show the pressure correctly. "The atmospheric or reverse-valve is added to the boiler to guard against the effects of external pressure, as the steam cools after the fires are let down. It is sometimes considered sufficient for this purpose to open the safety-valve, but a reverse-valve is now generally added: it consists of a valve fitted against a seat, and opening inwards, being kept shut by a small lever and weight." Steaming Capabilities. [Extract from a letter from Mr. Frederick Robinson, Mate of the " Cyclops," to Mr. Weale, dated Alexandria, 17th April, 1842.] "The steam can be raised from cold water, temperature 60°, in about 75 minutes, with about rds of a ton of coals. After two years' experience, it is found that 24 hours' consumption averages 24 tons, or about 1 ton per hour, equal to about 7 lbs. per horse power per hour, and which, at an average rate of 8.4 miles per hour, leaves the expense of steaming about 3s. 10d. per mile, if we use the following prices for the consumable stores, viz., Coals, per ton...... Oil, per gallon... Tallow, per lb..... Oakum, per lb. 30s. 48. 5d. 3d. And these are believed to be the average Mediterranean prices. "The maximum speed attained by Cyclops is thought to have been 11 knots, though there has never been more than 10-5 registered in the log-book; this is when in good trim for speed, having about 100 tons of coal on board, and drawing between 15.3 to 15.6 forward, and from 15.9 to 16:3 aft. The average speed in moderate weather, with between 200 and 300 tons of coal on board, and drawing 15.9 forward, 16.6 aft, or thereabouts, is 9 knots. "As a frigate she is most serviceable; and, but for her want of power, could scarcely be improved upon. It is principally in towing heavy ships that her want of power is most conspicuous: her burden, as before stated, is 1,195 tons, while her engines are but 320 horses. Her rate of towing the line-of-battle ships and small threedeckers, (like the Princess Charlotte,) has been 6 or 6.2 knots per hour; and the expense of taking them to sea, out of Malta Harbour, has been estimated at 37. 108. each vessel. The fuel used has been chiefly Scotch and Welsh, and, when mixed, has been found to burn well; but if used separately, it has been found that there is a difference of nearly one pound per horse power per hour in the consumption, the Welsh being the most extravagant. It is a difficult matter to arrive accurately at the wholesale consumption, for it is a common complaint in the service, that the quantities of coal stated in supply notes from the dock-yards always greatly exceed any estimate or measurement for consumption; there being always a great difference between supply and expenditure." Expense of Steam Frigates. "As the expense of the year 1841 may be interesting, I shall here quote it: it will give an idea of the estimate for a steam navy. "During the year 1841 the Cyclops' services were as follows: Mr. Galloway gives first a "history of the invention." He finds traces of it in the "Machines et Inventions approuvées par l'Academie Royale des Sciences," 1727-1731; also, in a work by Paucton, "on the Theory of the Screw of Archimedes," 1768; and, what is more remarkable, produces documentary evidence of its having been successfully applied in the British navy as early as 1802, (by manual power of course,) to move ships of war in action. The in ventor of this British application of it was a Mr. John Shorter, of Wapping Wall, from whom Mr. David Napier obtained, at a subsequent period, the particulars disclosed in the following extract "Mr. Napier having made some experiments with a screw which he believed to have originated with himself, showed it to various persons, and thereby became acquainted with Mr. Shorter's previous trials; and having found that person to be living in Southwark, he called upon him at his residence, and was shown a large collection of models of the screw propeller applied in the dead wood, the quarter, the bow, at the vessel's sides, and, in short, in every possible position. The screws also were varied in their form, consisting of one continuous thread, of two, three, and four threads, of mere vanes, like a windmill, and of a single arm. Indeed, Mr. Napier states that he appeared to have contemplated every possible arrangement, and that his models comprised most of the modifications now before the public. He showed Mr. Napier a number of experiments in a reservoir he had constructed for that purpose in his workshop, by which it appeared that the best performance arose from a single blade or arm projecting from an axis, and this seems to have been the form he used in its adaptation to the vessels referred to in the certificates. The position in which he fixed them is doubtful, but the impression is that they were placed one in each quarter, the axes passing through stuffing-boxes." A Marestier, in his memoir on the steam navigation of the United States, 1824, described several methods of propelling on the principle of the screw, which had been either tried or proposed." year later, Mr. Samuel Brown, the inventor of the gas vacuum engine, applied a propeller on the principle of the screw to a small boat (at the bow) which he had fitted with his new engine "With this vessel several trips were made, principally between London Bridge and Battersea, though occasionally below bridge. In this vessel, we are informed, there were sometimes as many as thirty persons carried at once, at an average speed of 6 to 7 miles per hour. On one occasion, she passed through Battersea Bridge at the same time as the Diana Richmond boat, and passed the current in less time than the Diana, though the latter is stated to have been 20horse power." Tredgold mentions several other parties by whom the idea of screw propul sion was at different times taken up, tried, and abandoned. But though all these facts show that the propelling power of the screw is "no new discovery," they prove at the same time that it had never been turned to any practical account down to the recent period, (1836,) when Mr. J. P. Smith brought it so prominently under the notice of the public, by his patents, and by his successful experiments with the Archimedes. Mr. Galloway pays a wellmerited tribute of praise to the Messrs. Rennie, for the active and public-spirited part which they have taken in carrying out Mr. Smith's plans to a successful issue. After describing some experiments which Mr. Smith had made with a small model vessel, he thus proceeds— "The results of these trials were so satisfactory as to lead to the formation of a Company under the title of 'The Ship Propeller Company.' The system of propulsion was, however, so unpopular among manufacturing engineers and scientific men generally, that it was some time before a manufacturer could be found to undertake an order to construct the engines and machinery for a large experimental vessel. Messrs. George and John Rennie (sons of the celebrated Mr. John Rennie) having, however, witnessed several trials with the little experimental vessel, and satisfied themselves that the invention was one of considerable promise, not only undertook the order, but contributed largely towards the necessary funds for carrying out the designs of the Company. It may be interesting to state that an invention which has been so far established that it has now become a subject of honourable competition to discover and apply the best arrangement of machinery and form of propeller, was, at that period alluded to with ridicule, or at best with expressions of regret that Messrs. Rennie should sport with their reputation and a Company with their capital in so hopeless a project. Such facts as these cannot be considered unworthy of notice, as showing the vast difficulties which oppose the prosecution of a new discovery and as proving that the successful practical application of a mechanical invention is attended with as many obstacles, and requires the possession of a quality of mind, almost as rare and as valuable to the community as the highest powers of invention."-p. 12. The machinery of the Archimedes is then very fully described,* and We observe, by the way, among this machinery; a "peculiar coupling," as it is styled, (Fig. 10,) which happens to be precisely the same as that VOL. XXXVII, a illustrated by a copper plate, on large scale, as well as by numerous wood engravings. After this come the experiments made with the vessel under the superintendence of Captain Chappell, with the results of which our readers are already familiar. The conclusion which Mr. Galloway deduces from these experiments is of rather an equivocal character. "As far as the question," he says, can be decided by such trials as we have enumerated, there can be no doubt that the performance of the screw has not been proved to be inferior to that of the paddle-wheel." Neither does Mr. Galloway think it possible to venture further in the way of opinion, "until two vessels of the same form and power (one fitted with the screw and the other with the paddle-wheels) shall have made several voyages, so as to test their comparative advantages in various kinds of weather." And a double experiment of this sort is, it appears, about to be actually made by the Admiralty-orders having been given to fit out the Rattler and Polyphemus, two sister vessels, with steam engines of precisely the same description and power-the former to be propelled by the screw, and the latter by paddle-wheels. Mr. Galloway favours his readers in the meanwhile with a mathematical investigation of "the relative value of the paddle-wheel and the screw, as regards the proportion which the propelling effect bears to the power employed." His conclusion on this head is "That when the paddle-wheel is at an ordinary immersion, and the slip equal in both systems, the advantage is somewhat in favour of paddles; but that when a vessel with paddles is deeply laden, or rolling considerably in a sea-way, so as to immerse the wheel beyond a certain dip, or when the screw is so constructed as to have less slip than the paddle, the advantage is in favour of the screw. And further, as a beam wind necessarily depresses the lee wheel so as to cause it to be disadvantageously immersed, a steamer with paddles can only partially avail of such a wind; while with the screw there is no such limitation, and canvass can made use of in Craven's Chimney Sweeping Machine, of which an account is given in our 990th Number, and which is there supposed by us to be new, and commended as a contrivance "of great ingenuity, and applicable to many other purposes." Not the first instance, by a great many, of a good thing being invented by two different parties, wholly independent of each other. N |