72 
Proceedings of Royal Society of Edinburgh. [sess. 
Very often this was conspicuous. Now and then I noticed one or 
more of the birds skimming for a half-moment at a time in a position 
which must have been so, viz., when they were hidden, or all but 
hidden, from us by a wave crest, the back or the descending side 
of which was towards the ship. As the waves were long and not 
high, it was only by keeping exactly in this position that a bird 
could remain invisible, or visible only partially and for a second or 
two at a time, as the wave varied in form a little, or as he rose and 
fell a little. 
“ It also frequently happened that two, three, or four of the birds 
were flying in close company, generally in single file. When they 
were thus flying close to the water, they occasionally ‘skimmed,’ 
and then after a few seconds began again to flap. And it was 
noticeable that they all made the change simultaneously , 
implying that they had simultaneously arrived at a suitable region.” 
At the end of a letter on other subjects, dated 10th March 1879, 
he says — “ I have made quite sure that the skimming birds follow 
the ascending wave-slopes as I had surmised.” 
These remarks of Mr Froude seem to make it clear that the up- 
draught of the advancing slopes of a ground swell in a calm may 
sometimes be a cause of soaring. It certainly seems to be the only 
cause which can account for soaring in a calm or very light wind ; 
on the other hand, it is a cause which can operate only when there 
is a large swell, and when the wind is either very light or not in 
the direction of the swell. 
Perhaps the most interesting feature of the letters consists in the 
analysis of the theoretical conditions of flight, and the paradox which 
thence results. For this paradox has an important bearing on the 
computation of air resistances in general, and any information which 
may serve to throw light upon it has a correspondingly wide 
significance. 
