VELOCITY OF THE MIC4EATI0N FLIGHT 



73 



activities, aud yet cannot have been created to no purpose what- 

 ever. Their sole jmrpose, therefore, is evidently to enable it to 

 perform those wonderful migrations — wonderful both as regards the 

 height at which they proceed, and the velocity with which they are 

 carried out. If birds were restricted during their autumn and 

 spring migrations to the same low strata of the atmosphere in 

 which they move during the rest of the year, such of them as 

 have to perform their migratory journeys early in the spring or late 

 in the autunm would in many cases be obliged, in consequence of 

 stress of weather, to let the proper period of their migration pass 

 without having been able even to make a start on their journeys. To 

 withdraw themselves from the disturbing influences which are apt 

 to prevail in these changeful lower strata, birds mount up into the 

 more elevated layers of the atmosj^here, in which more uniform 

 conditions prevail, and which are less subject to powerful meteoro- 

 logical disturbances. In this way, however, they reach elevations 

 at which the resistance of the air is so insignificant as to render 

 possible the astonishing velocity of flight developed during the 

 migration, while this velocity at the same time counteracts any 

 tendency towards sinking, a slight elevation of the anterior margin . 

 of the horizontal wing-surface being amply sufficient to eftect this 

 object. 



By considerations such as these we not only approach somewhat 

 nearer to an explanation of the velocity of the migrator}' flight 

 such as it has been proved to be, but we may also assume as an 

 established fact, that these migratory flights are possible, solely and 

 exclusively, under such conditions as prevail in paths at the im- 

 mense elevations discussed in the previous chapter. 



AUTHORS GARDEN. 



