The effect which this Machine is intended to produce, is analogous to several culinary or officinal processes that might be named. It is called rolling: but not in the same sense in which that word is used in manufactories, where rollers form or modify the body acted on. Here this body itself rolls between two surfaces moving different ways and receives from them the desired impressions, and this idea I have extended to screws; proposing to finish them on some metals and in some dimensions; and to rough them out in others. The Machine is represented in [figs. 6 and 7] of [Plate 19], where [fig. 7] shews the faces of the arcs A B of [fig. 6]. By the form and connection of the arms A C and B D, these arcs move opposite ways: and since they are grooved obliquely as shewn in [fig. 7], if a prepared cylinder of soft metal a, be put between them, and the handle C be sharply pressed into the position A E, the cylinder a will be made to roll, and the grooves of [fig. 7] be impressed on it so as to meet and form the screw in question. The only conditions are, that the arc B A be at least equal in length to the circumference of the screw, when finished; and that the grooves ([fig. 7]) be rightly sloped, and have the form intended to be given to the threads of that screw. It will occur of course, that the opening between the arcs at the point where the blank cylinder is introduced, must be larger than the distance between the arcs by the whole depth of the threads to be impressed: which therefore will begin to be formed at two opposite points the moment the screw a begins to roll. This however, might and would be otherwise, if it were thought best to form the arcs A B spirally; and let the deepening process be gradual: in which latter case another consideration would occur, namely; that the grooves themselves (see [fig. 7]) must diverge a little instead of being parallel, so as to permit the screw to lengthen as the pressure should displace a part of the metal. In all cases the upper surface of the grooves should be milled so as to lay hold of the soft metal, and insure the rolling motion: and should this material be hot-iron, the stroke should be taken in an instant, and the machine be kept cool by every proper method, in the intervals of working.
I need not add that this rolling process would be still easier performed, if the impressions to be made were circular and not oblique: such as beads, balls, &c. but these considerations I leave to my readers.
OF
A DIFFERENTIAL STEEL-YARD,
To weigh vast Weights with short Levers.
[Plate 19], [figs. 8 and 9], offers two representations of this Machine—one intended to shew its manner of acting, and the other one of its practical forms. By means of the first, ([fig. 8]) we may compare it with the common steel-yard; and even shew the latter as a part of the former. If a weight, or load to be weighed M, were suspended to the arm A B, and the counter-weight W, placed at the point C, of the arm A C, we should have a common steel-yard whose power would be as 5 to 1: for the arm A B is just 1⁄5 of the arm A C, and this is the principle on which steel-yards are commonly made. But instead of this, my steel-yard G E B D C H [fig. 8], is now infinitely powerful: so much so indeed, as to be infinitely useless. If millions of pounds were now to be suspended at P, they would not raise the weight W one tittle, for they hang entirely on the point of suspension A. But although the Machine is now useless, it can be altered in a moment and made both useful and commodious; only I thought its principle would be the better understood from being thus shewn in excess. To make it a useful and powerful Instrument, I only move the hanging bar D G, to a b; and the bar E B to c d, the lever b d being similar to that E G. In this state of things, the whole load P is found at the point o of the lever B H, (for the lever-arms c o and d e, and those e b, and a o are equal) and the power of this steel-yard is as the line A C to the line A o; that is as 20 to 1, instead of being as 5 to 1 which it before was. But this is not yet a powerful Machine; being chiefly intended to shew the principle on which it acts—and to prove that however small the distance A o, that distance, dividing the arm A C, gives the real power of the steel-yard. And supposing now the arm A C to be four feet in length, and the distance a D, B c, and A o, to be 1⁄10 of an inch, then the power of the weight w to raise (or weigh) the load P is as 48 inches to 1⁄10 of an inch, or as 480 to 1: so that if the weight w were 10lbs. this steel-yard would weigh 4800lbs. or upwards of two tons; and it is easy to see that this power can be almost indefinitely extended.