112 THE FLIGHT OF BIRDS 
has still a stretch of mgre than 800 miles of sea to 
fly over. Thus the New Zealand evidence, though 
it is not yet as complete as we could wish, justifies 
us in attributing to birds quite extraordinary powers 
of endurance. 
I have already pointed out that the rarefaction 
of the air at high altitudes makes it less resistant 
and less buoyant, an advantage without any draw- 
back. The migrant bird flying, say, some three, 
four or five thousand feet above the sea level, will 
not suffer from the attenuation of the atmosphere, 
for, since he travels with very great velocity, it will 
give him all the support he wants. But there is 
another fact that must be borne in mind which may 
make the achievements of migrant birds more 
credible. 
Homing Pigeons, as we have seen, make the best 
times as a rule when they have a tail-wind to help 
them. Migrant birds, on the other hand, very often 
have, it is said, the wind on the shoulder or blowing 
almost straight in their faces when they make their 
flights. But we must not without evidence jump to 
the conclusion that the wind at the altitude at which 
the birds are flying is blowing in the same direction 
as it does at our level. We have only to watch the 
clouds to discover that often at no very great height 
there is an upper current that is not following the 
same course as the wind below. Fifty years ago 
Mr. Glaisher made some investigations, by means of 
balloons, that threw much light on the subject. He 
found that, though the direction of the wind close 
to the earth was sometimes that of the whole mass 
of air up to 20,000 feet, yet at other times the direc- 
