256 Proceedings of Royal Soeiety of Edinburgh, [march 19 , 
5. Note on the Determination of Diffnsivity in Absolute 
Measure from Mr Coleman’s Experiments. By the 
President. 
6. On the Soaring of Birds : being an Extract from a 
Letter of the late Wilham Froude to Sir W. 
Thomson, of 5th February 1878, received after Mr 
Froude’s death. 
So much for sails. Now I want to make some suggestions, or 
suggest some queries, as to the shimming flight of birds, in reference 
to which a good deal of fresh observation has been possible during 
the voyage. 
You perhaps recollect that when the British Association was at 
Glasgow, you asked me to put into ‘writing, briefly, as a paper for 
your section, some remarks on this subject which I had made 
to you in conversation, but that, owing to my hasty departure to 
attend the trial of H.M.S. “ Shah,” I omitted to do this. 
I had better briefly recite the above particulars here in order to 
make more clear the bearing of the new observations we (I and 
Tower) have made. 
The view was that when a bird skims or soars on quiescent 
wings, without descending and without loss of speed, the action 
must depend on the circumstance that the bird had fallen in with, 
or selected a region where the air was ascending with a sufflcient 
speed. In still air the bird, if at a sufficient height, could continue 
to travel with a steady speed, using his extended wings as a sort of 
descending inclined plane, the propelling force depending on the 
angle of the plane and on the equivalent of “ slip,” that is to say, 
on the excess of the angle of actual descent compared with the 
angle of the inclined plane. The steady speed would be attained 
when the weight of the bird and the sines of the angle of the plane 
= the bird’s air resistance., including skin friction of wings, in fact 
one might say = simply the skin friction of the whole area, for the 
bird’s lines are fine enough to justify this statement, since there is 
no wave-making to be done, and indeed experiment shows that the 
