4 Rayleigii, llie Mecha?iical Principles of Flight. 



Some years ago I visited the north side of Madeira, where 

 cHffs, nearly 2,000 feet high, rise perpendicularly from the 

 sea. Being on the top of the cliff, we had difificulty in 

 finding a sheltered spot until we noticed that close to the 

 edge there was almost complete calm. Lying upon the 

 ground and moving only one's arms, it was possible to hold 

 a handkerchief by the corner so that a little behind the 

 plane of the cliff it hung downwards as in still air, and a 

 little in front of the cliff was carried upwards in the 

 vertically rising stream. A ball of crumpled paper thrown 

 outwards was carried up high over our heads. Of course 

 gulls and other birds found no difficulty in rising up the 

 face of the cliff without working their wings. During a 

 recent visit to India, I frequently watched the effect of 

 similar upward currents defiected by rocky fortresses 

 which rise from the plains. Kites could be seen to main- 

 tain themselves for minutes together without a single flap 

 of the wings. When this occurred, the birds were sailing 

 to and fro over the ivinchvard side of the rock. 



Wc now turn to the consideration of case (3). 



" In a uniform wind the available energy at the dis- 

 posal of the bird depends upon his velocity relatively to 

 the air about him. With only a moderate waste this 

 encrg}' can at an)- moment be a[)plied to gain elevation, 

 the gain of elevation being proportional to the loss of 

 relative velocity .squared. It will be convenient for the 

 moment to ignore the waste referred to, and to suppose 

 that the wh(jle energy available remains constant, so that 

 however the bird may ascend or descend, the relative 

 velocity is that due to a fall from a certain level to the 

 actual position, the certain level being of course that to 

 which the bird might just rise by the complete sacrifice of 

 relative velocity." 



In illustration of case (3) I instanced a wind blowing 



