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 



