422 



MUSCULAR MOTION. 



body slide over another; and adhesion the force, which unites two 



polished bodies when applied to 



Fig- 173. each other, a force, which is 



measured by the perpendicular ef- 

 fort necessary for separating the 

 two bodies. The more polished the 

 surfaces in contact, the greater is 

 the adhesion, and the less the fric- 

 tion; so that where the object is 

 merely to facilitate the sliding of 

 one surface over another, it will be 

 always advantageous to make the 



Composition of Forces. 



Fig. 174. 



Lever of the first kind. 



surfaces polished, or to put a liquid between them. 



A beam or rod of any kind, resting at one part on a prop or support, 

 which thus becomes its centre of motion, is a lever; The ten inch 



beam, PW, Fig. 174, 

 is a lever, of which F 

 may be considered 

 the prop or fulcrum ; 

 P, the part at which 

 the power is applied, 

 and W, the point of 

 application of the 

 weight or resistance. 



In every lever we distinguish three points ; the fulcrum, power, and 

 resistance; and, according to the relative position of these points, the 

 lever is said to be of the first, second, or third kind. In a lever of the 

 first kind, the fulcrum is between the resistance and power, as in Fig. 

 174 ; F being the fulcrum on which the beam rests and turns ; P, the 

 power; and W, the weight or resistance. We have numerous familiar 

 examples of this lever; the crowbar in elevating a weight; the handle 

 of a pump ; a pair of scales ; a steelyard, &c. A lever of the second 



kind has the resist- 

 ance W, Fig. 175, 

 between the power 

 P and the fulcrum 

 F; the fulcrum and 

 power occupying 

 each one extremity. 

 The rudder of a ship, 

 a wheelbarrow, and 

 nut-crackers, are varieties of this kind of lever. In a lever of the third 

 kind, the power P is between the resistance W, and the fulcrum F, Fig. 

 176; the resistance and the fulcrum occupying each one extremity of 

 the lever. In the last two levers, the weight and the power change 

 places. Tongs and shears are levers of this kind ; also, a long ladder 

 raised against a wall by the efforts of a man: here the fulcrum is at the 

 part of the ladder which rests on the ground; the power is exerted by 

 the man ; and the resistance is the ladder above him. 



Fig. 175. 



Lever of the second kind. 



