49 6 



PH YSIOL OGJCA L PH YSICS. 



[Chap. XLI. 



the 

 the 

 longer 



than the resistance, and has consequently always the 

 advantage. It is the lever of power, though, as in 

 the first order, the power must always move through a 

 greater distance than the weight. 



The third order of levers is shown in Fig. 206. 



The power is between 

 the weight and 

 fulcrum. Here 

 weight has a 

 arm than the power. 

 Let w be distant 4 feet 



Fig. 206. Lever of Third Order. n ,, 



irom F, and p 2 feet, 

 and let w = 10. The moment of w is 40. Acting 



o 



through 2 feet a power of 20 is necessary to yield the 

 same moment. Therefore, with these distances the 

 power must be more than double the weight to raise 

 it. Here, therefore, the weight has the advantage. 

 But it is evident from Fig. 207 that the weight 

 moves through a much greater 

 distance (from w to w') than the 

 power (from p to P'). A small 

 movement of the power will, 

 therefore, give a good sweep of 

 the weight. This lever is, 

 therefore, a lever of velocity ; 

 the weight passes over a con- 

 siderable distance in a short time. 



We shall see, in the chapter on Animal Mechanics, 

 (chap, xliii.) how the muscles, bones, and joints of the 

 body can be classified under such a system of levers. 



The balance is another illustration of the prin- 

 ciples applicable to parallel forces. This is particu- 

 larly well shown in the Danish balance (Fig. 208). 

 It consists of a steel arm with a fixed weight P at 

 one end. At the other end is a hook carrying a scale 

 pan. The arm is supported fr.m a beam resting on 

 the edge of the ring-shaped oody F, which is the 



Fig. 207. Lever of Third 

 Order. 



