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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 or 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 99 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, i i i, 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 fron 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, WM. BADDELEY.

London, March 29, 1834.

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 be 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

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and let a thread be fastened to the lever at 2 inches above the fulcrum, as shown in fig. 2, where a b is the plane, ed the lever, e 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 rotation 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

Fig. 3.

Fig 4.

a

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 di rection opposite to the pull, the distance of 9 inches, because the leverage will in this case be as 9 to 1; the dotted lines show the 3 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 direction of the upper thread, the distance of 19 half inches, or 94 inches, being as the sum of the two radii to their difference; and the common fulcrum will be at a point equi

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 two threads being fastened to the pulleys, and endless thread be made to pass around two riggers at the ends

of the path of the pulleys, one side of which endless thread takes a single turn round one of the pulleys, and the other side of the thread a single turn around the other pulley, the endless thread being turned by one of the riggers, or by a power applied in any other manner, will draw the pulleys with the same proportional velocity, in the direction of the thread passing around the smaller pulley; see fig. 5, where a is the endless thread, b the riggers, c the pulleys, d the lower radius, e the continuing place of the fulerum of the leverage.

Fig. 5.

a

"In Mr. Saxton's model at the National Gallery of Practical Science, Adelaide-street, West Strand, the pulleys are about as 8 to 9, consequently the sum 17 to the difference 1, expresses the real velocity of the carriage relative to that of the drawing cord; and a horse at his most effective speed of two and a half miles per hour, attached to an endless rope passing round pulleys in those proportions, would draw a carriage attached to the common axis at the rate of 42 miles per hour."

Now taking Mr. Saxton's elevation (or that of Mr. Hawkins'), viz., that of Shooter's-hill, which is assumed to be the highest at which any rail-road would be of practical utility, and which rises in the ratio of one in eight, I find, by calculation and experiment, that it requires twelve times the power of propulsion at that acclivity, that would suffice upon level ground.

Whatever it is-be it what it maythat a single horse power can propel upon a level rail-road, the same weight would require twenty-five horse-power to propel upon Mr. Saxton's plan. Now, it is obvious that, if this be the fact, it would • require twelve times twenty-five, or 300

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horse-power, to propel any given weight at the above elevation. In order to be distinctly understood upon this point, I will re-state it in other words: What one.. horse will perform upon a level rail-road, 300 horses will be required to do upon a plane at the elevation of Shooter's-hill, on Mr. Saxton's plan. How, it may be asked is this immense power to be attained, or, if attained, at what cost? Could any benefit result from it? In my opinion there could not. It follows, then, upon an ascent equal to that of Shooter's-hill, the "locomotive differential pulley" loses all its supposed virtue, and becomes worse than abortive: for, be it remembered, that Mr. Saxton's plan is chiefly recommended upon the ground that it will supersede the necessity of cutting through hills.

It is my object to show, that Mr. Saxton's plan is founded in ignorance of the principles of mechanics. The interest, as well as the honour, of science renders this exposition necessary; but I have no personal feeling whatever in the ques

tion.

An

My model represents a double railroad, the inner one being for the truck (which I shall describe presently), and the outer one for the vehicle to be propelled. The former, as well as the latter, runs upon four wheels. The dif ferential pulley consists of one larger and one smaller pulley fixed together, and both revolving on the same axis. endless rope, that is to say, a rope like a lathe-band, having no termination, passes over the larger pulley of the truck, and thence runs to its destination, viz., the point at which the truck stops; at that point it passes over a fixed pulley; after which, returning, it passes over the second, or smaller pulley of the truck; and, finally, again revolves over the first fixed pulley from which the truck commences its progress. To the rope we have been describing is attached a branch rope, which is fastened to a horse that draws the truck, in the same manner as barges are drawn on the banks of canals. Mr. Saxton proposes that the horse shall go three miles an hour-the vehicle to be propelled in that time thirty miles.

The vehicle to be propelled is placed over the truck, which has a balancecatch in fronts this catch strikes against a bar fixed to the vehicle, and the force