Manchester Memoirs, Vol. xliv. ( 1 899), No. 5. 7 



probably suffice to explain much of the sailing flight of 

 albatrosses and other sea-bird-;. 



Another way in which a bird may draw upon the 

 internal cncrg)^ of the wind has been specially discussed 

 b\' Dr. Langley {Siinthsonian Contrtbntwns, 1893), ^^ho 

 calls attention to the fact that the well Vinowngiistzness of 

 the wind, at any rate near the earth's surface, is under- 

 estimated in the usual meteorological records. The 

 differences of horizontal velocity involved in what are 

 commonly called gusts of wind imply in general vertical 

 motions also, but near the ground these latter may, 

 perhaps, be left out of account. The advantage which a 

 bird may take of the variations in the speed of the wind 

 is explicable upon the principles already applied, the inertia 

 of the bird playing in some sort the part of the string of a 

 kite. 



If // denote the speed of the wind at any moment, 

 and V the speed of the bird in the opposite direction, 

 both e.g., reckoned relativel)- to the ground, the available 

 energ}^ is measured by \{;v-\-iif. Suppose now that the 

 wind freshens, u becoming u-^cin, while v remains con- 

 stant. The increment of available energy is 



l{v + II + diif - ^{i> + uf ={v + !t) (in ; 

 or in time /, 



f{v + ii)dH (i). 



The speed of the wind being supposed to be periodic, 



and the integration being taken over a sufficiently long 



period of time, we have 



t 



f It du = Q ; 

 and thus the mechanical advantage may be reckoned as 



fvdu (2). 



