174 THE STRUCTURE AND LIFE OF BIRDS cuHap, 
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 isa great object. The arm isa 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 are required, but the muscles are there. 
