VII FLIGHT 175 
The wing-lever must find its fulcrum in the air. 
A strong brecze, or, better, a hurricane may give us 
a notion how this is done. The air, which, when 
still, seemed to offer no opposition to our progress, 
becomes, when moving at a great pace, an obstacle 
through which we must shove our way with effort. 
It makes no difference whether it is an actual wind 
that opposes us, or whether it is one existing only 
relatively to ourselves, being produced by our own 
rapid travelling. A bicyclist if he rides fast on a 
calm day is retarded by a breeze due to his own 
velocity. If there is a light breeze ahead, this may 
be doubled by the pace at which he rides, so that 
what to a pedestrian is hardly a breath of air is 
magnified by the bicyclist into a wind. The resistance 
of the air, then, increases if we move through it more 
quickly. But this is only a very vague statement, 
that gives but little idea of the facts. Some experi- 
ments were made by Newton showing that in many 
cases the resistance of the air increases as the square 
of the velocity. These experiments depend on the 
fact that a body, when let fall, gains in velocity for 
some time, after which it maintains a uniform pace. 
Nothing but the resistance of the air can check a 
progressive increase in velocity, and when the pace 
becomes uniform it is clear that the resistance of the 
air is exactly equal to the weight of the body falling. 
Newton took glass globes of equal size, but unequal 
weights, corresponding to the figures I, 4, 9, 16. 
These he let fall from a height, and measured their 
velocities when each had settled down to its uniform 
pace. The velocities were in proportion to the 
