FLIGHT 27it 
Upward currents of air are to be met with, when there is a 
wind, on the windward side of a sloping mountain, or a house, 
or the sail of a ship, or as a result of the inequalities of surface 
produced by waves on the sea. For example, Gulls may be 
seen to soar for a prolonged period of time in front of or above 
cliffs on the shore when the wind comes from the sea. This, 
however, cannot be taken as an example of soaring proper, seeing 
that Gulls can only remain suspended in the air without loss of 
relative horizontal motion under special conditions which do not 
apply in the case of the typical soaring birds. Gulls are not seen 
to soar except under conditions which point toward upward cur- 
rents, being a very obvious explanation of the phenomenon. In 
the case, on the other hand, of typical soaring birds, such as those 
named above, soaring is observable under conditions and at heights 
where there is no sufficient reason to assume that upward cur- 
rents exist. For example, Eagles and Adjutants are seen to rise 
continuously by soaring for miles above the surface of the ground 
or sea. To explain such cases as being due to upward currents 
would, we are of opinion, require a good deal more evidence of 
continuous upward currents of air from the plains or seas than is 
at present available. That the direction of the wind even at great 
heights, and above a comparatively smooth sea or plain, is by no 
means always parallel with the surface of the globe is more than 
probable ; but we know of no reason for assuming that the upward 
currents are sufficiently predominant over the downward currents 
to justify us in looking on the former as capable of explaining 
observed facts as to soaring. In other words, the theory of upward 
currents of the air as an explanation of sailing or soaring flight 
requires more support in the way of facts than have been so far 
produced. 
The theory of wnequal velocity of the air at different heights as an 
explanation of soaring is based on the fact that the motion of a 
wind is retarded by the surface of the earth, so that considerable 
variations in velocity may and do occur at different heights above the 
ground. The matter is put very clearly by Hubert Airy,! although 
we are indebted, in this country at all events, to Lord Rayleigh for 
having first formulated it. Airy says: “Suppose a bird at the 
highest point of one of its gyrations, when it has mounted against 
the wind and is wheeling to one or other side preparatory to the 
descent with the wind... . Let us regard the air at the level 
of the bird, at this turning-point, as still. Then, relative to this 
point, the lower strata of air have a horizontal velocity in the 
opposite direction to the wind (as perceived on earth); and the 
bird in falling apparently down the wind will really be meeting 
stronger and stronger adverse currents, and when it has reached 
1 Nature, xxvii. pp. 534 and 591. 
