XIV MIGRATION 361 
would avoid the great heat of equatorial regions and 
seek further south a milder climate more similar to 
that to which they have become accustomed. But 
though this seems likely, and though in the case 
of some species definite evidence has been obtained, 
it is unsafe to represent it as universally true. 
The Height at which Migrants Fly.— Their Cries. 
There is no doubt that birds usually mount to a 
great height when about to start on a long voyage. 
Homer Pigeons, when liberated, circle upward, in 
order, apparently, to survey the country and take 
their bearings. But exact information as to the 
altitude reached is very difficult to obtain, since it is 
impossible to take trigonometrical observations of a 
passing object. The best evidence available has been 
procured through the ingenuity of some American ob- 
servers. They have watched birds through telescopes 
focussed upon the moon and have calculated the 
height at which they were flying. One of these 
observers, Mr. F. M. Chapman, proceeded on the 
assumption that the least distance at which a bird 
could be seen was one mile (nearer than which it 
would not be in focus), and the greatest, five miles. 
The apparent altitude of the moon was calculated 
at ten minute intervals, in order to discover the 
angle formed by the telescope with the horizon. 
When these two facts had been ascertained—(1) the 
distance, within certain limits, of the bird from 
the observatory, (2) the angle at which the tele- 
scope inclined upwards—it was easy to calculate 
