INTRODUCTION. 9 



it not for this arrangement. When bones which 

 meet and form a joint are set at an angle, then of 

 course there is less chance of concussion, and so we 

 find less ' cartilage' needed. 



12. — As there are three forms of levers, we must 

 be careful to remember this, and we shall close 

 this somewhat dry and relatively uninteresting 

 lesson by a few remarks on the mechanical lever, 

 and compare it with the animal lever. 



The lever is an unyielding bar, (represented in 

 the animal by bone,) capable of free motion about 

 a fixed axis, called the 'fulcrum.' To this un- 

 yieldiug bar, ' power' is applied, (which in the 

 animal lever is represented by muscle and its tendon). 

 Lastly, we have the weight, resistance, or obstacle to 

 be overcome by the power. 



If the fulcrum {F) be placed between the power 

 (P) and the weight ( W), so that when the power 

 sets the lever in motion the weight and the power 

 describe arcs, the concavities of which are turned 

 towards one another, the lever is said to be of the 

 first order, (see Fig. 3, A). If the fulcrum be at 

 one end, and the weight be between it and the 

 power, so that weight and power describe concen- 

 tric arcs, the Weight moving through less space, the 

 lever is of the second order (Fig. 3, B). And if, 

 the fulcrum being still at one end, the Power be 

 between the Weight and Fulcrum so that the Power 

 and Weight describe concentric arcs, the Power 



