LEVER. This simple but invaluable me-. chanical instrument consists merely of a bar of wood or metal, by fixing one point of which called the fulcrum, a pressure at the end more distant from the fulcrum is made to counter balance a larger pressure at the nearer end ; or, if both ends he equally distant from the fulcrum, equal pressures are made to balance each other.
The first explanation of the lever was given by Archimedes, and in a simple and correct form. It assumes two principles ; firstly, that when a system is in equilibrium, the state of rest will not be disturbed if additional pres sures, such as compensate each other, and would by themselves produce no motion, be introduced or removed ; secondly, that, when a weight is made to rest by being attached to an immoveable point (say it is suspended by a string), the point or pivot of suspension undergoes a pressure equal to the weight of the system, whatever may be the form of that system, or the dispositions of its parts. Thus, a cylindrical or prismatic bar of uniform ma terial will necessarily rest if a pivot be passed through a section in the middle of its length, since there is no reason why it should pre ponderate on either side ; now, divide the bar into any two parts, and, from their middle points, suspend weights equal to the weights of the parts ; also apply counterpoises of equal weights at the same points, by means of strings passing over pulleys ; the equilibrium will then be undisturbed; take away from the system the parts of the bar and the counter poises (which are equal and opposite to one another), and the equilibrium will subsist be tween the remaining weights, which are those first suspended. The points of suspension
being in the middle of the lengths of the two parts are at distances from the pivot (at the middle of the whole length of the bar), which are inversely proportional to the suspended weights.
This may be briefly illustrated thus. When a straight and uniform bar is supported on a central pivot, if weights on either side of the pivot keep the bar exactly horizontal ; then the weight on one side, multiplied by its dis tance from the pivot will he equal to the other weight similarly multiplied by its distance. If a weight of eight lb. balances another of six teen, the former must be twice as far from the pivot as the latter. It is on this principle that the Steelyard, and other kinds of weighing machines, act.
In English treatises on mechanics it is cus tomary to call one of the pressures which ba lance on a lever, the power, and the other the weight. Levers are thus distinguished as of the first, second, or third kind, according as the fulcrum, the weight, or the power, is in the middle.