The bar rq is an iron lever, which in the above diagram is of the second kind, having at r a pin, inn, like that of a balance, turning iu two steel rings on a pillar of stone sunk in the ground, or of iron resting on the bottom of the box ; a steel pin, pq, formed also like that of a balance, with its edge upwards, passes through the bar PQ; and upon this the edges of the prisms at cd and c'd', as shown in the section.
The platform supporting the carriage which is to be weighed, and which is represented at re x in the section, has below it four iron feet, of which two appear at x and Y; and the under surfaces of these feet are formed with hemispherical cavities which rest on the points of four conical steel pins at t, u, ', u', on the two lever-frames; consequently the weight on the platform pressing at these four points, the prisms at cd, c'd', are forced down upon the pin pq; and this last then presses , i down the extremity q of the lever eq. his lever itself may be made to act as a steelyard ; or, by a rod or chain, the extremity Q may be connected with one arm of a balance or steelyard above the machine ; in either case the weight of the carriage may be ascertained.
It is evident, from the nature of the lever, the pressure of the plat form itself being balanced by a constant weight at q, that if w represent the weight of the carriage, w. . will express the pressure at 4, ae PQ or the weight by which that of the carriage is determined ; and this is independent of the power obtained by a steelyard which may be eon ac 4 rq a t I rx con nected with Q. If = and — 1 a weight equal to 10 pounds 1 at q' will balance a weight equal to 1 ton upon the platform.
The Spring Balancc is a machine in which the elasticity of a spring of tempered steel is employed as a means of measuring weight or force. The namo is moat commonly applied to machines for the former pur pose ; those employed to ascertain the muscular strength of men and animals, the amount of power required to move a carriage or a boat, or any other force applied in the form of a pull, being called dyna mometers. [DYNAMOMETE11.] One of the simplest kinds of spring-balance is that which, when employed as a weighing-machine, is known as the spring or pocket steelyard. It consists of a helical spring formed by bending a steel wire spirally round a cylindrical mandril or axis, so as to form an extensive series of convolutions, This spring is placed in the interior of a tube of brass or iron, closed at both ends ; one end of the spring abutting against the plate which closes the lower end of the tube. A
rod, having a hook or loop at Its lower extremity, to hold the article to be weighul, passes through a hole in the bottom of the tube, and up the inside of the spring. At the upper end of this rod is a small plate, which slides; up and down like a piston in the tube, and ruts upon the upper or free end of the spring; thereby causing it to collapse when a heavy body is attached to the hook at the bottom of the alitling.texl. The machine is supported by means of I s or ring attached to the upper end of the tube; and the extent of the motion of the spring, and consequently the weight of the body suspended from it, are indicated by the degree to which the rod is drawn out of the tube. For this purpose a graduated scale is engraved upon the rod; the divisions indicating the extent of compression pro duced in the spring by the application of known weights. Several spring-halances on the same principle are made for various purposes. That known as Salter's balance has a brass plate attached to the tube or cylinder, within which the spring is enclosed, and a vertical alit through the plate and tube. A scale is engraved on the face of the brass plate, and the weight is indicated by a pointer which moves up and down with the spring, with which it is connected through the vertical slit in the tube. Martin's "index weighing-machine' acts upon the same principle, but has a circular dial-plate and a revolving pointer or index, resembling the hand of a clock. On the axis of the index, but at the back of the dial-plate, is a toothed pinion, which is turned by a straight rack attached to the vertical rod, which rises and falls with the spring. The index remains in a vertical position when the balance is unloaded, and deviates more or less from it when a weight is attached to the book. One advantage of this construction is that the point of the index traverses a much greater space than the spring itself, so that a small movement of the spring becomes readily discernible.