238 Experimental Philosophy. [Lecture 16. 



D ; in this case the power must be equal to three 

 pounds, in order to support the weight. 



Disadvantageous as this kind of lever appears, 

 it is upon this principle the human arm is con- 

 structed ; for the muscle which moves the arm, 

 and which is inserted in the bone below the 

 elbow, may be considered as the power, which 

 you see is placed between the weight to be raised 

 by the hand and the prop, or place where the 

 muscle is inserted above. To compensate for 

 this disadvantage, these muscles are made unusu- 

 ally strong, and we may judge of their immense 

 power by the weights which athletic persons are 

 enabled to wield. The same power exerted 

 only on equal terms ought to raise a weight of 

 ten thousand pounds. 



II. The wheel and axle (fig. 98.) is the next 

 in order of the mechanic powers. The power is, 

 in this machine, applied to the circumference of 

 the wheel, and the weight to be raised is fastened 

 to one end of a rope, of which the other end 

 winds round an axle that turns with the wheel. 

 This instrument is more commonly used with a 

 handle : thus, to wind up a common kitchen 

 jack, I turn the handle, which coils the cord 

 round the axle in the middle : to wind a bucket 

 from a well, I do the same thing ; to wind up 

 my watch, the same : the handle in all these is 

 in the place of a wheel, and the farther this 

 handle is from the centre, the axle, on which the 



