54 THE BIKDS OF HELIGOLAND 



Humboldt, however, subsequently added that the bird jjrobably 

 attained even greater altitudes than those obtained by calculation, 

 and states that on Cotopaxi, 13,578 feet above sea-level, he had seen 

 it soaring above him at such a height that it apjjeared no larger 

 than a small black ' dot.' This height cannot with certainty have 

 been less than 30,000 feet. From calculation we find that an object 

 eleven feet in diameter would require to be at more than double 

 that elevation before it disappeared from view; and, according to 

 Humboldt's statement, eleven feet maj^ be assumed as the average 

 expanse of the wings of the Condor during flight. An idea of the 

 almost incredible distance at which objects may be seen in the 

 clear mountain air of these regions may be formed from further 

 reports of Humboldt, according to which he was able with the 

 naked eye to observe Bonpland, who, clad in a white mantle, was 

 riding along the dark face of a clift' at a horizontal distance of 

 84,132 feet. 



Practical observations made in the open air in Heligoland 

 have yielded similar results. The oyster bank, which lies to the 

 east of the island at a distance of 22,000 feet, is frequently visited 

 by vessels ; if one of these on a clear day were to display a flag of 

 the same breadth as the expanse of wing of the Condor, it would 

 not only be at once seen from the island, but in a favourable light 

 an eye of ordinary keenness might even be able to recognise its 

 colours — say, blue, red, and white. 



One is justified in assuming, that in the high clear mountain air 

 in which Humboldt's observations were made, the distance at which 

 the bird was visible was at least as great as that at which a flag of 

 a breadth equal to that of the bird's wings could be seen in a low, 

 mist-laden atmosphere, like that round Heligoland. Hence there 

 can be no doubt that 40,000 feet is only a low estimate for the 

 height of the Condor's flight above sea-level. 



Eesults such as these tend to make one hesitate in pronouncing an 

 opinion as to the altitude of the flight of a bird, such as, for instance, 

 the Grey Yulture, with a wing-expanse of ten feet, one of which 

 Dresser watched through a good pair of field-glasses, until the bird 

 before disappearing from sight, was reduced to the size of a small dot. 



In comparison with observations like these, my own experiences 

 in this place in regard to this subject will appear insignificant. The 

 main result deducible from them, however, is, that, with few excep- 

 tions, the altitudes of migration are far beyond the limits of vision of 

 the sharpest eye. Cf course, the various species differ as much from 

 each other in the height of their migration as they do in regard to 

 the direction in which the movement proceeds; but in the case of 

 the vast majority of migrants, both on arrival and departure, the 



