FARROW'S MACHINE FOR SILVERING LOOKING GLASSES. The large silver medal and five pounds have been presented, by the Society of Arts, to Mr. George Farrow, of Silverstreet, Golden-square, for a machine of his invention for silvering looking-glasses. The common silvering-table for lookingglasses, is a slab of stone, ground to the most perfect degree of evenness, and placed in a frame, so that a certain degree of obliquity can be given to it. All round the margin is a gutter, through which, at one corner, a hole is made, so as to allow the escape of the mercury, when the plug that closes the hole is removed. It is this corner which is the lowest, when the oblique position is given to the table, in order that the mercury may run to it from the other parts of the gutter. On the silveringtable is spread a sheet of tin-foil, of the same size as the glass, or rather a little larger; a fluid amalgam of tin is then poured on it, and spread over its surface with a brush till it adheres; more mercury is then poured on, till it stands about a quarter of an inch deep over the tin-foil. The plate of glass being previously made quite clean, is then slid, gently and steadily, from a sheet of paper, just dipping below the surface of the mercury, but avoiding to touch the tin foil, for fear of tearing it. When the glass is fairly over the tin foil, the table is placed a little oblique, by means of a rack; the mercury now runs into the gutter, and the glass subsides on the tin foil. The whole surface of the glass is then covered with leaden 7 lb. weights, having cloth at the bottom. By this pressure, at the end of twenty-four hours, the silvering is so firmly adherent to the glass, that the weights may be removed, and the glass raised up in a sloping position, to allow the mercury to drip off, till the silvering has become quite hard. Mr. Farrow's improvement, as described in the last part of the Transactions of the Society of Arts, consists in dispensing with the loose leaden weights, and in producing the required pressure by means of screws. It is attended with the following advantages:-First, all hazard of breaking the glass during the application of the pressure is avoided; when loose weights are used one will sometimes slip out of the hand of the workmen, and, falling on the glass, will break it. Secondly, the plate, as soon as the pressure is made by means of the screws, can be tilted up, even in a vertical position, so as to expedite considerably the drip of the mercury from the silvering; an operation which is manifestly impossible when loose weights are employed. Mr. Farrow himself has hitherto applied his invention only to small plates for dressing table-glasses; but Mr. Wheeler, a manufacturer of looking-glasses, has applied Mr. Farrow's apparatus, with some modifications to plates, 48 inches long by 29 inches wide. In the accompanying figures, 1 is the top view of one end of a large stone bed; 2 is a section of the same; ab the stone slab; c d its frame, containing the usual channel for the mercury; e one of the end supports, on which the bed may turn for the purpose of being tilted; the middle strengthening bar f, which serves for an axis, is placed a very little on one side, to make the side d, at which the slope is given, always preponderate, that side usually resting on one ог more screws, by which it is lowered or raised again. The dotted lines in fig. 2, show that side of the bed as lowered, and resting on a block. The upper and under sides of the frame cd are made quite parallel to fit the hooked ends of the long clamps gg, which slide from one end to the other. The clamps are fur. nished with little plates h h; these project inwards for the clamps to stand or slide on, when the screws are loosened. They are also, with the. under hooks, made sufficiently wide, as shown in the end view, fig. 3, to prevent the clamps from falling on one side. A sufficient number of these clamps is provided to range over the bed, about 1 foot apart; and the screw-holes in one clamp are made to be opposite the intervals in the next, as shown in fig. 1, in order to distribute the pressure more equally over the surface of the glass. The clampingblocks iii, are made of wood faced with leather-they hang on the screws loosely, so as to rise and fall with them, and allow of being placed in any position. Fig. 4, one of these blocks separate; they are about 7 inches long, and the screws are eight inches apart. The clamps are usually all drawn to one end of the bed to be out of the way, and to make room for the glass plates being slid on, and need only be taken off when the largest glass is to be silvered, and then replaced to give the pressure. Fig. 5 is an end view, or elevation, of a portable bed, for silvering small glasses, and is Mr. Farrow's original invention; it has wooden sides kk raised above the bed, in which the several wooden clampingbars g g slide. Here two screws only are used to each bar; and the battens that hang on their lower ends are in one piece. This, with the glass on it, is placed by hand in any required position. The glasses being quite clean for silvering, the faces of the battens, or clamping-blocks, iii, never take any dirt whilst in use; and as they always remain pendant, with their faces downwards, when out of use, they keep clean, so that the glasses are not liable to any scratches from them. HYDRAULIC PROJECTOR. Sir, I will neither occupy your valuable columns, waste my own time, nor insult the understandings of your readers, by refuting, in detail, the arguments adduced by Mr. Witty in this day's Mechanics' Magazine, in support of his watery project; his letter is a singular compound of truth and error, not easily unravelled, or briefly answered. He has admitted the important fact, that he has not yet constructed the machine described in your 544th Number; when he has done this, and finds its performances equal his expectation, it will be high time for him to explain to N. C. (vol. xx. page 301) "the cause of such extraordinary results." Mr. Witty says, "the action of the mechanism your correspondents do not find fault with:" truly, the mechanism may be much improved upon, but a fig for such improvements in a machine founded on false principles. I have no doubt that when Mr. Witty can get the water to be so obliging, as to jump twenty feet high from a conical fifteen-inch tube, plenty of contrivances will be forthcoming to give due effect to so much condescension. Mr. Witty wishes to know the extent of my last visit to Newcastle, with particulars of my experiments. When there, lately, I had the full use of all my senses, common sense included, but I never stated that I made any experiments! This is another of Mr. Witty's fancies. Whenever Mr. Witty constructs a machine like the one he has described, and obtains the result he calculates, I guess he will have proved the existence of such a pretty considerable difference between the laws of gravity, motion, and atmospheric pressure, in this latitude and in Staffordshire, that I shall feel it incumbent on me to proceed thither directly, and institute suitable experiments to establish the novel and marvellous peculiarities of this (then) wonderful county. When this comes to pass, you shall hear more about the matter, from, Yours respectfully, London, March 29, 1834. WM. BADDELEY. SAXTON'S DIFFERENTIAL PULLEY, AND ITS APPLICATION TO LOCOMOTIVE PURPOSES. Sir,-Having visited the experimental railway laid down near Gloucester-gate, Regent's-park, for the purpose of exemplifying Mr. Saxton's plan for propelling vehicles on a rail-road, by means of what he calls his "locomotive differential pulley," and heard the lecture thereon by his friend Mr. Hawkins; but being far from satisfied with the results of my observations on the occasion, I determined to inquire further into the subject. I therefore constructed a model to answer that purpose, and am now prepared to demonstrate that Mr. Saxton's plan can never be made to realize the expectations formed of it. But, before doing so, it may be as well to give your readers some account of this differential pulley; which I cannot do better-more fairly, at least, to the inventor-than by quoting the description of it which Mr. Hawkins read to the Society of Civil Engineers, and which was afterwards published in the Repertory of Arts: "I beg leave," says Mr. Hawkins, "to premise (what is familiar to every engineer and mechanician) that any radius of a wheel or pulley may be considered as a simple lever. "This investigation may be pursued under three cases. "Case 1st.-Let the lower perpendicu lar radius of a vertical wheel of 40 inches diameter, resting on a horizontal plane, be considered as a lever 20 inches long; see fig. 1, where a represents the horizontal plane, b the wheel, c the lower radius or lever. Fig. 1. "Let the lowest end of the radius where it meets the plane, be the fulcrum of the lever, and let a thread be fastened to the lever at 2 inches above the fulerum, as shown in fig. 2, where a b is the plane, cd the lever, c the fulcrum, ef the thread. If that thread be pulled horizontally the distance of I inch, it is obvious that the top of the lever will be moved in the same direction the distance of 10 inches, as shown by the dotted lines. "Now, let the top of the lever represent the axis of a wheel or pulley of 20 inches radius, and the pulley itself in ›rotations on the plane be considered as a continued succession of such simple levers, with fulcra progressing at the same rate, it will be evident that if a thread be fastened to, and wound around a pulley of 18 inches radius, then the common axis will move forward 10 inches for every inch motion of the thread; and the progressing fulcra, forming the periphery of the wheel, will keep pace with and remain perpendicularly under the axis, the velocity of which to the speed of the thread, being as the radius of the wheel is to the difference of the two radii; in this case 10 to 1. "Case 2d.-Let the thread, mentioned in case 1st, be held fast so as to constitute it a fulcrum of the lever at two inches above the plane; and let another thread c g, fig. 3, be fastened to the bottom end of the lever and pulled in an opposite direction, the distance of 1 inch, the result will be, that the top of the lever will be moved in a direction opposite to the pull, the distance of 9 inches, because the leverage will in this ease be as 9 to 1; the dotted lines show the position to which it would be pulled. "Case 3d,—Let both threads be pulled in opposite directions at the same time, and each drawn the distance of half an inch, then the top of the lever will be moved in the 19 half direction of the e upper thread, the distance of or 94 inches, being as um of the tes the common fulcrum will be at a point equi radii to their difference the dand distant, and at a right line between the points of traction of the two threads; see fig. 4. "Now let the threads be fastened to and pass round the peripheries of two concentric pulleys united together, of 18 and 20 inches radii respectively, the threads being pulled in opposite directions, will cause the common axis of the pulleys to proceed in the same direction as the pull of the upper string, with a velocity of 19 times the speed of the threads; being like the simple levers befors shown, as the sum of the two radii is to their difference, But if, instead of tw two fastened to the pulleys, and threads being thread be made to pass around two riggers at the ends |