Auk 



360 Porbcs, Concerning the Flight of Gulls. [july 



abreast oi her to the windward side, the somewhat sheltered lee 

 sido being persistently a\ oided. On a le> el with her upper deck or a 

 little above it. the,) wore generally and rather evenlj distributed — 

 although more thickly in places than in others all the waj from 

 her stern to amidships, some keeping within a yard or two oi the 

 rail, others thrice that distance off, still others fifty or more yards 

 out o\ cr the water." Ho also occasionally saw "one of them leave 

 the rest. ami. going two feet to their one, forge ahead oi them all 

 perhaps to the hows oi the steamer ami beyond, yet without once 

 beating its wings." 

 Mr. Brewster notes that when the birds soared during the height 



oi the gale their wines were held back ami with a "downward trend 

 of the flight quills." He suggests that "the wind constantly tills 

 the concave wings oi the eliding gulls much as it does the sails of 

 close-hauled vessels and with similar results but with this essential 

 difference: that whereas its force is exerted for the most part later- 

 ally on the vessels' sails and opposed by the side thrust oi their 

 keels or centreboards in the water, it must have chiefly a lifting 

 effect on the wings of the gulls and be counteracted by the weight 

 oi their bodies hearing downward. Hence We may infer that in 

 the ease of these birds forward movement is the resultant of two 

 component forces, that oi wind am) of the attraction oi gravitation." 

 He then refers to an article by 0. F. Tydeman in which by " e\- 

 tensrv e use oi abstruse mathematical calculations " the phenomenon 

 is analysed. Mr. Brewster then says that lie dissents from Tyde- 

 man's conclusion " that birds gliding to windward depend for means 

 oi propulsion largely if not wholly on uplift afforded h\ powerful 

 ascending currents oi air such as must always rise above a vessel 

 when hea\\ wind is striking against and deflected from, her sides." 

 He says that for a time he favorably considered this view, adding: — 

 " But 1 dismissed it altogether from my mind after repeatedly seeing 

 birds hundreds of yards behind the steamer, or fifty or more yards 

 to one side always the windward one) of her. or even well in ad- 

 vance of her. gliding on set wings in precisely the same manner and 

 quite as ceaselessly as those which hung about her flanks. It 

 seems inconceivable that her presence or movement could ha\e 

 caused vertically rising currents oi air to be regularly maintained 

 at such distances from her as those just mentioned, or that they 



