Manchester Memoirs, Vol. xiiv. ( 1 899), No. 5. 



THE WILDE LECTURE. 

 V. The Mechanical Principles of Flight. 



\\y the Ivt. lion. Lord Ravi^kigii, F.R.S. 



Pelivrycd Fchntary r^lh, igoo. 



The subject under discussion includes both natural 

 and artificial flight. Although we are familiar with the 

 flight of birds, there are many interesting questions which 

 arise in connection with natural flight, and some of them 

 are yet very obscure. 



In still air a bird, being heavier than the fluid displaced, 

 cannot maintain his level for more than a short time without 

 working his wings. In this matter the vicarious principle 

 holds good. If the bird is not to fall, something must fall 

 instead of him, and this can only be air. The maintenance 

 of the bird thus implies the perpetual formation of a down- 

 ward current of air, and involves therefore performance of 

 work. Later we shall consider more particularly how this 

 work is applied ; but a preliminar\' difficulty remains to be 

 discussed. It is well known that large birds, such as 

 vultures and pelicans, are often observed to maintain 

 their level for considerable periods of time, without flapping 

 or visibly working their wings. On a smaller scale, and 

 in more special situations, sea-gulls in these latitudes 

 perform similar feats. This question of the soaring or 

 sailing flight of birds has given rise to much difference 

 of o[)inion. Few of the naturalists, to whom we owe the 



April 26t]i, igoo. 



