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 
PEPy 
Fie. 1. 
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 
B2 
