48 THE MIGRATION OF BIRDS 



Most of these observations were at first made by 

 accident ; birds were seen through astronomical 

 telescopes passing across the face of the moon or 

 sun ; but recently this method of observation and 

 several ingenious plans of measuring height and 

 speed have been made use of expressly to study 

 migration. W. E. D. Scott, at Princetown in 1880, 

 thought that by shape and size he could even recognise 

 two species, Chrysomitris tristris and Quiscalus pur- 

 pureus, which passed across his field of vision at a 

 height of at least half a mile above the earth (43). 

 In 1888 Mr Frank M. Chapman published an account 

 of similar observations ; he calculated that the birds 

 passed at distances varying from one to five miles, 

 and that their altitudes must therefore have been 

 between 600 to 1000 feet, and 3000 to 15,000 feet. 

 He adds an important note : "A number of birds 

 were seen flying upwards, crossing the moon, there- 

 fore, diagonally, these evidently being birds which 

 had arisen in our immediate neighbourhood, and 

 were seeking the proper elevation at which to con- 

 tinue their flight," but the direction of flight of most 

 of the birds which were observed was parallel to the 

 earth's surface and southerly. The average height 

 was certainly far above the inferior limit (13). 



Mr F. W. Carpenter, reviewing these astronomical 

 calculations, says that Verey compared the apparent 

 size of birds with lunar features, and considering 



