POWER AND WEIGHT. 55 



far greater mechanical advantage than the present flexor of the hock; but 

 it could not bend that joint to anything like the same extent, because 

 muscles cannot contract to more than about two-thirds of their normal length. 

 Besides, such an arrangement would be extremely inconvenient for every- 

 day work, and would increase the liability of the limb to injury. Although 

 there is, therefore, a very large expenditure of muscular force in the action 

 of the levers of the limbs ; there is an equally large gain in flexion and 

 extension, and consequently in speed. This arrangement, also, enables the 

 body to be made of a compact form, and to be suited to its surroundings. 



Directions in -which the Power and Weight respectively 



Act. — In the theoretical levers which I have given {see Figs. 6, 7, and 8), I 

 have assumed that the power and weight acted at right angles to the lever, and 

 that they were consequently parallel to each other. In the actual levers 

 (those of the hock) which I have taken into consideration, we may see that this 

 is not the case. I may mention that the nearer a force is to being at right 

 angles with its lever, the greater is the mechanical advantage at which it will 

 work. If, in a lever of the first order, for instance, we have the power and 

 weight, as in Fig. g, acting in directions which are not parallel to each other, 



I 

 I 



a'r 



F ' 



W P 



Fig. 9. — Lever of First Order with Directions of Power and Weight 

 Oblique to each Other. 



•such forces (if the lever be in equilibrium) will then be inversely propor- 

 tionate to the length of the perpendiculars drawn to their respective direc- 

 tions. Thus in Fig. 9 we have P : W : : Yd: Ye. We therefore see that W, 

 which is nearly at right angles to a b, acts much more advantageously than 

 P, which is in a much more oblique direction to it. This would be equally 

 true in the other two kinds of levers. I need not stop to prove the foregoing 

 •well-known mechanical law, as its solution can be found in any book 

 < n elementarv statics. 



