vii FLIGHT 175 



The wing-lever must find its fulcrum in the air. 

 A strong breeze, or, better, a hurricane may give us 

 a notion how this is done. The air, which, when 

 still, seemed to ofTcr 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 1, 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 



