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DEMONSTRATIONS OF APPARATUS 217 trating the equilibrium and parallelogram of forces. I pur- posely select this, as probably almost the last experiment that it would occur to anyone to demonstrate by this method. As is well known, we arrange two pulleys H and K, over which hang weights p and Q, drawing two flexible cords from the point A, from which depends another weight E by the line A L, the lines A B and AC measured from any point D on the per- pendicular by lines D B and D c parallel to the cords, giving the ratio of the weights P and Q, and A D that to both of the weight E. Simply using rather thicker flexible cords than usual, all this will be projected on the screen with the greatest facility, and on a large scale ; while by supporting in a clamp a plate of glass just in front, and in the plane, of the cords, the parallelogram may be actually traced out with a black brush, appear- ing on the screen as drawn, and the law verified by measurements. It has been said that some little modification is required to use this method with the lantern; but it is very slight. We merely want to pro- Fm vide as small and intense a radiant point as possible. When the radiant in the lantern is an arc- lamp of moderate power, sufficiently sharp projections will be obtained from the arc direct, removing all lenses from the lantern front. An incandescent lime is too large for sharp- ness, however, and the best method is to place on the front the little ' pencil' attachment shown in fig. 95, p. 173, removing the parallelising concave lens. The chief part of the rays will then be passed as a conical pencil through one of the holes on the front, the hole being reduced till the desired sharpness is obtained. The range of the lantern is immensely extended by thia method of projection, which has two advantages. The image on the screen is not inverted; and it can be made of any size