338 
WILD NORWAY. 
suggest; but it seems certain that some great facts 
or factors governing migration have been overlooked, 
or yet remain to be discovered. 
The facts that migration is never observed in day¬ 
light, and that known flight-lines cross great altitudes 
(ten thousand feet to twenty thousand feet and upwards), 
point to these aerial movements 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 
convexity of the earth, but not for mountains or other 
obstructions, the figures work out roughly as under— 
Altitudes. Tangential extent of vision. 
25,000 feet ... ... ... 225 miles. 
30,000 feet . 245 „ 
Thus, at the first-named altitude, birds would, in 
daylight, see England long before losing sight of 
Norway: while at the higher level, when crossing Spain, 
they would see Biscay before sinking Africa. 
In doubting whether migration has been seen in 
