THE HEIGHT AND SPEED OF FLIGHT 53 



hour, the bird will cover a distance of 30 miles in 

 one hour, though the force exerted by the bird is 

 the force necessary for 20 miles in a calm. Con- 

 versely, if a 10-mile air current meets it, it will 

 unconsciously be carried only 10 miles. If the speed 

 of the bird is the same as the opposing force of the 

 wind, it will remain stationary ; I have seen ducks 

 in a blizzard rise head to wind and fly rapidly, 

 making no progress but maintaining their position 

 over the water, to which they dropped again when 

 the storm passed. Some black-headed gulls on the 

 same water did not attempt this manoeuvre, and 

 in a few seconds had vanished down wind. The 

 swimmer, in a swiftly-flowing river, may hold himself 

 in position so long as he can swim at the same rate 

 as the stream he is contending with, but he cannot 

 make headway if the speed of the water exceeds his. 

 He may, however swiftly the stream moves, swim 

 in any direction, but his actual progress will be down 

 stream ; if he aims to swim directly across, his real 

 course will be diagonal. 



The fact that birds fly in any direction in a wind, 

 and when at low elevation pay little heed to the 

 direction of the wind, when the breeze is light, 

 simply means that they can fly faster than the 

 medium they are in ; if the medium travels faster 

 than they do, they will be carried in it to their ad- 

 vantage or disadvantage. 



