LEVER, the most simple and common, but; at the same time, most important of the seven mechanical powers, consists of an inflexible rod—straight or bent, as the ease May be—supported at some point of its length on a prop rn. which is called the fulcrum, and having the weight to be moved and the power to move it applied at other two points. In the accompanying illustration (fig. 1, a), AB is the lever, F the fulcrum, A and B the points of applica Fig. 1, a. tion of P and W,the power (or pressure) and weight respect ively. If the arms AF and.BF be equal, the power P and the weight W must also be equal to produce equilibrium; if the arm of the power, AF, be I g-er than the arm of the weight, BF, then, to produce equilibrium, the power P must be less than the weight W, and vice verso; if AF be double the length of BF, then B, to produce equilibrium, must be half of W; and, generally, as is shown in the elementary treatises on mechanics, the power and weight are in the inverse ratio of their distances front the fulcrum. This is equally true for straight or bent levers; but (fig. 1, b) the distance of the power and weight from the fulcrum is not, in all cases, the actual length of the arms, but the lengths of perpendiculars from the fulcrum upon the directions of w the power and weight. This principle holds good, what Fig. 1, b.
over be the relative positions of the power, weight, and fulcrum; and as there can be three different arrangements of these, we thus obtain what are called " the three kinds of levers." The first kind (fig. 2) is where the fulcrum is placed between the power and the weight; the balance (q.v.), spade (when used for rais ing earth), see-saw, etc., are examples of this; and scissors and pincers are examples of double-levers of the same kind. Levers of the second kind (fig. 3) are those in which the weight is between the power and the fulcrum; examples of this are the crowbar, when used for pushing weights forward, the oar—the water being the fulcrum, and the row-lock the point of application of the weight—wadi the wheelbarrow; and of double levers of this kind we have nut-crackers as an example. In levers of the third kind (fig. 4), the power is betWeen the weight and the fulcrum. Fishing-rods, whips, umbrellas, and most iustruments used With the hand alone, are levers of the third kind, and shears, tongs, etc., are examples. of double-levers of this class. It is evident that, to produce
equilibrium in levers of the first kind, the power may, according to the ratio of the lengths of the arm, be either greater or less than the weight; in the second kind. ' • , it must always be less; and in the third -- kind, always greater. This is expressed -- A.:.- ' in technical phrase by saying that the first kind of lever gives a mechanical , / rage or disadvantage (see MECHANICAL POWE1tS), the second always gives a mechanical advantage, and the third always a mechanical disadvantage. Le- • , very of the second kind, having the same '. iL:\ "'"', A. 1 3, .1. 1 ''' 2 .' ,,, . , • ' ' mechanical advantage, are, when worked ' .:# .
...... - - • by man, twice as powerful as those of the .021...
first kind, because in the one ease he uses his muscular force as the power, in the other ease only his weight. Levers of the third kind are used when velocity, or a large extent of motion, is required at the expense of power, and we consequently find this form much used in the structure of the limbs of animals. The structure of the human arm (fig. 5) is a very good example of this; the fulcrum is the socket (C) of the elbow-joint, the power is the strong muscle (the biceps) which passes down the front of the humerus, and is attached at 'A to the radius (see Num); the weight is the weight of the forearm, together with anything held in the hand, the two being supposed to be combined into one weight acting at13. By this arrangement, a large extent of motion is gained, by a slight contraction or extension of the muscle. .
When a large mechanical advantage is required, this may be obtained, without an inordinate lengthening of the lever, by means of a combination of them (as in fig. 6).
Here the levers have their arms in the E ratio of 3 to 1, and a little considera a.. tion will make it plain that a power (P) of 1 lb. will balance a weight of 27 lbs.; but in this instance the particu ^ lar defect of the lever as a mechanical power shows itself prominently; for if the weight has to be lifted 2 in., the Fig. 6. power requires to be depressed (2x27 or) 54 in.; and, as the extent of sweep of the power cannot be largely increased with out inconvenience, the advantages of this machine are confined within narrow limits.