THE "GREAT BRITAIN," AND SCREW PROPELLING. its diameter out of the water, and the other half was acting efficiently at the same time; whereas under similar circumstances, with such a cross sea, the leeward paddle-wheel would have been immersed, probably above its shaft, while the windward wheel would have been completely out of the water; the strain upon the engines, in such a case, was very prejudicial; but in the Great Britain he never noticed any variation in the working of the engines, not even when she was struck with the heavy sea which had injured the bow. He Captain Sir CHARLES NAPIER inquired, whether the Great Britain steered well, and whether it was not found she had a tendency to fall off to leeward in a cross sea? should have supposed that the action of the screw propeller being so entirely in the stern, it would act upon the ship like sculling a boat. Captain HOSKEN replied, that the Great Britain steered extremely well; and that there was not any tendency to fall off to leeward. The action of the screw could not be correctly compared with that of a scull upon a boat; in that case, the power acted entirely upon the stern; but with the screw, the power was exerted in the direction of the shaft, up to the engines, in the centre of the ship, and by a simple arrangement it could be carried on even up to the bow. He was of opinion that the leeward paddle-wheel had not much power to keep a vessel up to the wind; it was so close to the ship's side that its leverage was not considerable. Sir CHARLES NAPIER thought the principal danger of the screw propeller was in running before the wind in a heavy sea. If struck by a heavy wave, the sternpost and the propeller might be carried away together; as also in case of getting on shore, the screw would not be so efficient in clawing off shore as the paddle-wheels would be. Captain HOSKEN said it was evident that the propeller was not easily injured, for since his arrival in the Thames he had found, coiled round the shaft, nearly 9 fathoms of chain cable, which had been apparently torn away from the mooring of a buoy, in coming up the river. Mr. J. MILLER said, one point of importance in favour of the screw was its not being affected by variations of immersion, arising either from the draught of water of the vessel, or from the rolling in a heavy sea. He had noticed particularly the difference of the speed of the engines, on board the Royal Mail Company's vessels, at the commencement and at the end of a voyage. ing, with a full complement of fuel, the paddle-wheels were plunged so deep, that At start 175 the speed of the engines, which ought on an average to be 17 strokes per minute, was reduced to 8 or 9 strokes, and at the end of the voyage the paddle floats had scarcely sufficient hold on the water. A vessel with a screw propeller would not be so affected. He thought also, that the screw was less liable than paddle-wheels to be injured by heavy seas. Captain HOSKEN was anxious to record clearly the points where he was satisfied the propeller was preferable to the paddle-wheel for steamers generally, but more particularly for the purposes of war and for Atlantic navigation. By using the screw, a great weight was entirely removed from the top sides and centre of the ship. The exertion of the power of the engines was transferred from the top sides and centre to the lower midship body, which was the strongest part of the ship. There was a saving of nearly one-half the weight. In the instance of the Great Britain, that ship was first intended for paddlewheels, which with all the appurtenances of beams, boxes, shafts, &c., were estimated at 180 tons. The weight of the propeller, the chain-wheels, shaft, chain, &c., might be taken at about 80 tons, and that weight was dispersed over nearly half the ship's length. When leaving port and the ship was deep, the propeller would exert its greatest power, when it was most required; paddle-wheels, on the contrary, when deeply immersed, would not allow the engines to exert their power. A steam-ship, with a propeller, answered the helm quicker, and steered easier, than a paddle wheel ship. A great point, also, was the superior efficiency of a screw-propeller ship under canvas, on account of the absence of the unsightly and detrimental paddle-boxes; possessing also the advantage of the sails acting with the engines, instead of injuriously to them, as with paddle-wheels. When the sails took effect, the ship heeled, or inclined to one side, the paddle-wheels consequently became too deeply immersed on one side, and had not sufficient hold upon the water on the other, manifestly wasting the power of the engines. The screw propeller was more easily disconnected from the engines than the paddlewheels, should it be required to save fuel, or if the engines were disabled, and the ship being properly rigged, a decidedly efficient sailing ship still remained, which in a paddle-wheel ship was not possible. The screw propeller was less liable to be damaged by heavy seas, or by shot, than paddle-wheels. Very recently, the West India Royal Mail Steam-packet Dee had one wheel quite disabled, by a heavy sea striking it; while the screw was nearly always so immersed as to be out of the reach of injury, either from waves or shot. As the relative merits of screw-propellers and paddle-wheels were of national importance, Captain Hosken felt confident no apology was necessary, for thus, as concisely as possible, giving his opinion before the Institution. If he might recommend any points for the consideration of the Members, it would be that they should exert their ingenuity to discover the best propeller, all circumstances being considered; as in his opinion it would be very difficult, if not impossible, to find a propeller that should be the best under every variety of circumstance. He con sidered the best method would be to multiply or reduce the speed of the propeller, as might be found necessary under different circumstances, but that, he was aware, would, in very large steamers, be difficult of attain ment. Sir CHARLES NAPIER agreed with Mr. Miller with regard to the disadvantages of the deep immersion of the paddles, particularly those of war steamers, where wheels were constantly plunged too deeply when they had their full armament and fuel on board. For fifteen years past he had urged upon the Government the necessity of paying more attention to the construction of their war steamers; for, in his opinion, there was not one really good steamer in the service; and he thought the Retribution, which was the last vessel finished, was not any improvement upon its predecessors. He objected particularly to the present construction of direct-acting engines, by which the working parts were exposed to injury from shot. He thought, that all the upper parts of the engines, and the naves of the paddle-wheels, should be made of wrought-iron, as in the case of being struck by shot, less serious injury would ensue than when they were made of cast-iron. He would suggest, also, whether it would not be possible to have tanks near the paddle-boxes, to be filled with water as the fuel was reduced in weight, and thus to keep the vessel at a uniform draught, so that the power of the engines could be always advantageously employed. Captain HOSKEN said, in reference to the points suggested by Sir Charles Napier, experience had shown that anything.cumbersome about paddle-wheels was bad for sea purposes, any machinery about them was difficult to be kept in order, and if the paddle-wheels were made to reef, when they were exposed to a heavy gale, or sea, they would assuredly lose a large portion of their paddle-floats. The suggestion of a contrivance to fill water about the paddle-boxes, in proportion to the fuel consumed, so as to keep an uniform dip of float-board, appeared not only objectionable, but it amounted almost to an impossibility. It was scarcely possible, even if desirable, to find space for 500 tons of water in a steam ship that might take that quantity of fuel as her sea stock; and doing so, would keep the ship in a long voyage continually groaning under a heavy burthen. He agreed with Sir Charles Napier as to the desirableness of an uniform dip of the paddle-board if it could be obtained; but if it was only to be arrived at by always carrying a heavy weight, it was better to continue the present plan, of starting deep and arriving light. Mr. GUPPY said, in answer to questions from members, that, at present, he believed the average speed obtained by vessels with screw-propellers was below that of paddlewheel steamers. A new screw of larger diameter and greater area of palms, was being made for the Great Britain, with a view to increasing the speed. It should not be forgotten, in the discussion, that a distinctive feature of iron vessels was their stiffness, and he conceived they were better calculated to withstand the shock of heavy seas than wooden vessels were. Four chains, weighing together about seven tons, were employed for communicating the power from the upper drum, upon the main shaft, to the lower drum upon the shaft of the propeller. They worked smoothly and without noise, and at present had not shown any tendency to wear, or to lengthen. From the form of the link, he conceived that the chains would only lengthen on the slack side, under any circumstances; and this would not affect their working, as the projecting ends of the links would, on the driving side, always fall into the recesses prepared for them, so that these recesses must be much worn, before the chains would ride out of their proper direction upon the drums. (To be continued.) INTENDING PATENTEES may be supplied gratis with Instructions, by application (post-paid) to Messrs. Robertson and Co. 166, Fleet-street, by whom is kept the only COMPLETE REGISTRY OF PATENTS. LONDON: Printed and Published by James Bounsall, at the Mechanics' Magazine Office, THE ATMOSPHERIC RAILWAY SYSTEM-IMPROVED PLAN. SIR,-I beg leave to forward for insertion in your pages (if acceptable) a plan for an atmospheric railroad, which seems to promise all the advantages of a locomotive railway, with more than its ordinary safety. I am acquainted with Mr. Pilbrow's plan, as far as your work gives it; but I must acknowledge I cannot perceive what is to prevent the piston outstripping the loaded carriages. The Dalkey Railroad, I think, possesses no arrangement for crossing on the level, nor any that allows of the trains stopping at intermediate stations, or going the whole length of the line without stopping, and in either case of travelling without attention from any party but the conductor. The "hot iron' of the Clegg and Samuda system is certainly no recommendation; and if it could be dispensed with, there would, doubtless, be a great advantage gained. I remain, yours respectfully, Description. B. C. G. It is proposed to have one centre main exhausting tube, for the whole length of the line, or such length as the power of the stationary engines will admit of, &c. The working tube to be in sections, as required, and connected with the main as shown in the sketches.-W is the main o, the main continued under a roadway; B, the working tube; F, sliding valve, shutting off communication with the atmosphere; while, at the same time, by means of the wheel R, the valve G opens a communication with the main exhausting tube, W, through the connecting pipe a. At a proper part on each side of the leading carriage would be fixed the wheels X, to travel the slides dd, (the ends, d d, being on their centres at fig. 1, r r,) thus raising the rack e upon the wheel R, as required, to act upon the valves F and G. Fig. 3 is an inside view of the valve F; H is a plate, ground true on one side, to slide over the opening upon the leather I, (which might be united to the casting by india rubber) and not quite flush with the inside of the tube. The top, v, of the plate H, is intended to fit close under the leather of the long valve. Fig. 4 shows the long valve raised by the piston slide L. Fig. 5 is a side view of the piston, and the slide L, which is intended to lift the valve; showing also the place of connexion, E, with the carriage. It will be observed, that should any accident prevent the opening of the valve H, the carriages would proceed notwithstanding, without the possibility of incurring any damage. Any shock that might arise from the percussion of the piston against the valve, as it is but a light weight, would be obviated by a spiral spring inside the hollow piston. To ensure the safe passage of the piston from one section to the other, it is proposed to enlarge the ends of the tubes, say to 4 in. inside diameter more than the size of the piston, as shown in fig. 2, and gradually to decrease the diameter until it assumes its proper size, and fits the piston, &c. a short distance before the longitudinal valve commences at C, fig. 1 and 2; which valve is continued some 6 or 8 feet, unconfined by any pressure of the atmosphere, till it reaches the section valve, thus permitting an easy entrance for the piston slide under the long valve, and providing against the ingress of air in front of the piston at the opening of the section valve. The long valve may be effectually protected from the weather by a kind of tarpauling cloth, as indicated by the dotted line, fig. 4. The other valves are necessarily protected by their peculiar construction. All being thus secured from injury by the weather, there does not appear anything to prevent them from wearing well-if they can be made air-tight. Some good self-lubricating plan would probably effect that object, and under that idea, it is proposed that soft grease should exude from conveniently-sized holes, n n, round the fore part of the piston, and on both sides the piston slide, as shown at one side marked n nn. The hollow piston being provided with another hollow cylinder inside, as far as y, and the space between the two, holding the lubricating matter, a piston, v, aided by a slight spiral spring at s, so that the matter may be kept up to the surface, the frictional demand would induce the required supply. By this means it is supposed that the tube and valves would be sufficiently and equally greased; for the suction and connection valves, when once done, would supply themselves by their own action from the waste from the tube. Thus the long valve, by bearing on the convex edge, CASE IN RAILWAY ENGINEERING. 179 is at A and the radius the direct distance from A to D. This inference leads us directly to the method of resolving the problem; for it is a well-known principle in geometry, that when two straight lines are drawn from two points any how placed without the circle, to another point in the circumference, the sum of these lines is a minimum, when they make equal angles with the tangent at the point of concourse, and as a consequence, with the radius of the circle to which the tangent is drawn. With the three given distances construct the right-lined triangle A B C, making A B=25 miles; A C-31 miles, and B C 22 miles; then with the radius A D=12 miles and centre A, describe the circle MD N. Let D be the point in the circumference sought, and through the point D, draw the tangen RD S, intersecting the distance A C in the point H. From the given points B and C to the point of contact D, draw the straight lines B D and CD; then if the angles B DR, CD S, or A D B, A D C, It appears from the conditions of the problem, that the distance of the town A from the station of supply at D, is given in magnitude, but not in position as regards the direction of the distances from A to B, and from A to C respectively; for it is stated in the enunciation, that the engineer was at liberty to lay it down in any position he pleased, limited, of course, to the angle contained between the direct distances of A from B, and of A from C. From this we infer, that the station D is situated in the circumference of a circle, of which the centre are equal between themselves, the straight lines D B, D C, when taken conjointly, shall be less than if they were drawn to any other point in the circumference; |