174 THE STRUCTURE AND LIFE OF BIRDS chap. 



absolutely fixed point, though, of course, the lever 

 would be improved if it could be made so. The third 

 kind of lever in which the power is applied at a point 

 between the weight and the fulcrum is not often used, 

 because it does not economise labour. We have 

 an instance of it in the treadle of a sewing-machine, 

 where the force required is so slight, that economy 

 is unimportant. Wasteful as it seems to be, this third 

 kind of lever is the common one in the bodies of 

 animals. All three classes are represented, but ex- 

 amples of the first and second are comparatively rare. 

 Consequently there have been people who have main- 

 tained that the human body is a clumsy machine made 

 on antiquated and unscientific principles. Such an 

 idea shows the danger of a little knowledge. When we 

 use a lever, we wish to move a weight with compara- 

 tively little effort, however much we may lose in the 

 speed and amount of the movement. In the levers of 

 the body rapidity is a great object. The arm is a series 

 of levers of the third order, and by their help it can be 

 drawn in quickly, then shot out again to deal a sudden 

 blow. If we try to hold out a weight at arm's length, 

 we then find the weak point of levers of this order. 

 To economise effort with them you must apply the 

 power near to the weight. In the case of the arm, 

 we should require a biceps, springing, as now, from 

 near the shoulder but attaching near the wrist, and 

 this, besides other inconveniences, would entail great 

 slowness of movement. In a bird's wing the leverage 

 which aims at moving a weight with great rapidity is 

 to be seen in its greatest perfection. Very powerful 

 muscles arc required, but the muscles are there. 



