258 Proceedings of Boy at Society of Edinhurgh. [march 19 , 
from hollow to crest. The speed of such waves would he from 24 
to 27 knots. 
Under these conditions the birds seemed to soar almost ad libitum 
both in direction and in speed. ISTow starting aloft with scarcely, 
if any, apparent loss of speed. Now skimming along close to the 
water, with the tip of one or other wing almost touching the surface 
for long distances, indeed now and then actually touching it. The 
birds were so large that the action could be clearly noted by the 
naked eye even at considerable distances ; but we also watched 
them telescopically and assured ourselves of the correctness of our 
observations. The action was the more remarkable owdng to the 
lightness of the wind, which sometimes barely moved our sails, as 
we travelled only 5 knots before it, by help of the screw. 
After long consideration the only explanation of at all a rational 
kind which presented itself was the following, which indeed pre- 
sents the action of a vera causa, and one which was very often 
certainly in accordance with the birds’ visible movements, though 
it was often also impossible either to assert or to deny the accord- 
ance ; and anyhow the question arises. Is the vera causa sufficient % 
I will try to trace its measure. 
When a wave is say of 10 feet in height and say 10" period (a 
case near enough to ours to form the basis of a quantitative illustra- 
tion) the length of the wave from crest to crest is just 500 feet, the 
half of which space, or 250, the wave of course traverses in 5", 
and assuming the wave to be travelling in a calm, it must happen 
approximately that during the lapse of this 5" the air which at 
the commencement of the interval lay in the lowest part of the 
trough has been lifted to the level of the crest, or must have risen 
10 feet, so that its mean speed of ascent has been 2 feet per second 
(10 feet in 5 seconds). And since (as is well known) the maximum 
speed of an harmonic motion is ^ times, or nearly IJ times its 
mean speed, it follows that all along the side of the wave at its 
mid-height the air must approximately be ascending at the rate of 
3 feet per second, and if the bird were so to steer its course and 
regulate its speed as to conserve this position he would have the 
advantage of a virtual upward air current having that speed. 
