GLIDING 3 



air that makes flight a possibility. The resistance 

 of air to a moving body may be little, may be 

 great. That it may sometimes be considerable 

 many a bicyclist has found out to his cost when 

 he has tried to double and redouble his speed. 

 There comes at last a time when the " yielding 

 air " almost refuses to yield. In fact the resistance 

 it offers to a body moving through it increases as 

 the square of the velocity. Of the ingenious experi- 

 ment by which Newton proved this I must give 

 a brief account. He took glass globes of equal 

 size but unequal weights, corresponding to the 

 figures 1, 4, 9, 16. These he let fall from the dome 

 of St. Paul's and measured the velocities when 

 they had settled down to a uniform pace. Since 

 there was no gain or loss of velocity, the resistance 

 of the air must have been in each case equal 

 to the weight of the falling globe. But it 



turned out that the relative velocities corresponded 

 to the figures 1, 2, 3, 4, whereas the weights of the 

 globes are represented by the squares of these 

 numbers, viz. 1, 4, 9, 16. From this he concluded 

 that the resistance of the air increases as the square 

 of the velocity. Recent experiments have shown 

 that Newton's law is not absolutely accurate. Up 



B 2 



