Mr. Abel Chapman's Observations 343 



"The fact that migration is never observed in daylight, 

 and that known flight-lines cross great altitudes (ten 

 thousand feet and upwards), point to these aerial move- 

 ments being performed at heights, and perhaps at speeds, 

 that have never fully been realized. Assuming that 

 migration is carried on at altitudes of twenty-five or thirty 

 thousand feet, it would follow that the conditions of air- 

 resistance, buoyancy, etc., in a rarefied atmosphere, would 

 there be entirely altered. What those altered conditions 

 may be or their precise effect on flight, it must be for more 

 scientific pens to define ; but, in my view, it can only be by 

 virtue of some such changes, assisted by the option of 

 selecting favouring wind-strata, that the feebler and short- 

 winged travellers can make their distances. 



"An obvious advantage of high-level travelling would be 

 the extension of the field of view. Allowing for the con- 

 vexity of the earth, but not for mountains or other obstruc- 

 tions, the figures work out roughly as under 



A Ititiides. Tangential extent of vision. 



25,000 feet 225 miles 



30,000 feet 245 miles. 



" Thus at the first-named altitude birds would, in day- 

 light, see England long before leaving Norway ; while at 

 the higher level, when crossing Spain, they would see 

 Biscay before sinking Africa. 



" In doubting whether migration has been seen in actual 

 progress, I do not wish to be taken literally, for I have my- 

 self occasionally witnessed it, but always in that limited 

 and relative sense in which exceptions go to prove rules. 



" Thus, in Southern Spain, in spring, I have observed 

 whole hosts of Swallows to appear suddenly, as from the 

 clouds. The plains, the marshes, the vineyards, or mud- 

 flats swarm with them. One night they roost in thousands 

 under your thatched eaves and in the trees hard by. 



