246 THE HAUNTS OF LIFE 



words, the bird has to exert itself to keep up 

 in the air. In the stroke of the wing it has 

 to displace to thrust away from itself down- 

 wards and backwards a mass of air bigger 

 than its own body. The resistance the air 

 offers to being thrust away is what keeps the 

 bird up. 



If we watch birds we see that the first 

 strokes of the wings in lifting the body cost 

 them much. A Great Northern Diver cannot 

 rise off the ground at all, though by getting 

 some weigh on by swimming rapidly it can 

 launch itself clean out of the water. We often 

 see a cormorant taking a little run along the 

 rock to get up speed enough to enable it to rise. 

 Even after it has got launched in the air it 

 often strikes the water again and again. Birds 

 like to start from a vantage-point, and a pigeon 

 gets woefully tired if it has to start many 

 times in quick succession from the ground. 

 But note the important point: Ce n'est que le 

 premier pas qui coute; once the bird has got 

 up a certain velocity in the air, the effort re- 

 quired to keep itself up becomes beautifully 

 less. Sir Isaac Newton showed that it decreases 

 in proportion to the square of the velocity, 

 and this is a very important fact. If there is 



