68 
Proceedings of Royal Society of Edinburgh. [sess. 
descend. And the work which can be done per time-unit by 
animals, when taxing their strength in given degree, is said, I be- 
lieve, to bear in general a fairly constant proportion to their total 
weight ; in other words, to be an approximate constant when stated 
in the form of the equivalent speed of lift of their own weight. 
Hence conclusion (4) above is in accordance with the fact that 
the larger flying birds are comparatively few, and that the largest 
birds do not fly at all. 
The value of this supposed approximate constant, viz., the speed 
at which animals in general are capable of continuously lifting their 
own weight for a long time at a stretch, if estimated from the reputed 
“ horse-power ” (or equally from the reputed man-power), would be 
about 30 ft. per minute, or *5 ft. per second. If, then, the same 
relative power-capacity may be assigned to birds as we have estimated 
for the average animal, we might have concluded at the outset, and 
without the aid of the mathematical reasoning which has been given, 
that the upward air current of 3 ft. per second ascribed to the passage 
of the waves, would suffice several times over to enable any birds to 
soar that are able to fly in still air. 
On the other hand, equation (8) above, if interpreted by any such 
numerical values as would be used in any ordinary mechanical 
problem of the kind, gives a value for V 1 (a 1 + oq) much greater than 
3 ft. per second, and we are thus confronted by a serious paradox. 
Mr Froude took for his albatrosses W = 20 1bs., A = 22 square 
feet (figures presumably obtained from the officers of the ship).* For 
P, F, and r, he took (to minimise the paradox) as the most favourable 
values which he thought might be conceivably justified, P = — 
2i 17 
F — 2 |^qqq > r = 1 '5. These values for P and F are approximately 
the values fairly well established for water, multiplied by the specific 
gravity of air ; F not increased on the score of the greater viscosity 
of air, but P doubled on the score of advantage that might con- 
* These figures give ^=’91. Memoranda of Mr Froude’s include weights 
A 
and measurements afterwards obtained, which show much higher values. 
Recent measurements of my own of an . albatross preserved in spirits at the 
W 
Museum of Zoology, Cambridge, give = 2*3. 
A 
