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 
