176 THE STRUCTURE AND LIFE OF BIRDS chap. 



numbers I, 2, 3, 4, but the resistance of the air was, 

 as we have seen, equal to the weights of the globes 

 which were as 1, 4, 9, 16, the squares of the numbers 

 which represent the velocities. Experiments at once 

 more elaborate and more accurate have been made 

 since. Professor Marey concludes from many made 

 by himself, that the resistance increases in a less 

 proportion for velocities between o and 10 metres per 

 second ; when 10 metres per second is exceeded, then 

 the rule of the square of the velocity under-represents 

 the rate of increase of the air's resistance. When 

 the speed attained is very great, in the case of a 

 bullet for instance, Newton's law does not hold at all : 

 the rate of increase of resistance altogether outpaces 

 the square of the velocity. The rate of movement of 

 a wing is comparatively moderate, so that here it 

 might seem that we should be safe in applying 

 Newton's law. There is liability to error, however 

 from another cause. A wing is very different from 

 the glass globes with which he experimented — it 

 presents a concave and irregular surface with rough 

 edges. Such an object passing through the air, which 

 is not a perfect fluid, but viscous, must, like an oar 

 forced through the water, produce eddies, and this 

 complicates the problem so much that our greatest 

 authorities confess that we know very little of the 

 resistance to a surface like that of a wing. It is 

 necessary to say this, since the rule " resistance of air 

 increases as the square of the velocity" is often 

 quoted as if it held true of all surfaces and all 

 velocities. Nevertheless it comes near enough to the 

 facts to be of great value, and probably when we 



