^^''igif ^"^1 Brewster, Notes on the Flight of Gulls. 89 



experimenting with severed wings stiffened by drying I have found 

 that when held firmly in the hand by their outer (i. e. bony) 

 edges and struck smartly downward they give one very decidedly 

 the impression that there is a resultant and by no means inconsider- 

 able level thrust which in the living bird would be directed forward 

 and might help materially to speed it on its way. If impetus may 

 be so derived it must be due to the leverage of the long, elastic 

 flight quills. Firmly attached at their bases to the rigid, bony 

 structure along the edge of the wing but free towards their tips to 

 be uplifted by impact on the air, these feathers perhaps act some- 

 what as a crowbar is made to serve by the mason when, after 

 inserting one of its ends under a heavy stone, he lifts at the other 

 end and thereby forces the stone forward over the ground without 

 raising it. Whether or not the suggestions just hazarded have 

 any value — a friend has characterized them as no less absurd 

 than the old time assumption that a man may lift himself by his 

 boot straps — there is, I think, little or no doubt that the wind 

 constantly fills the concave wings of the gliding 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 laterally on the vessels' sails and opposed by the side 

 thrust of 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 of their bodies bearing downward. Hence we may 

 infer that in the case of these birds forward movement is the result- 

 ant of two component forces, that of wind and of the attraction 

 of gravitation. 



The theory last stated is not novel of course. It has recently 

 been taken up and in certain ways effectively demonstrated and 

 supported by G. F. Tydeman ^ who, writing in French and making 

 extensive use of abstruse mathematical calculations accompanied 

 by diagrams to illustrate them, deals particularly and most inter- 

 estingly with the flight of sea birds. His observation of it has 

 apparently been so much more extensive than mine that I hesitate 

 to differ with him respecting any of his assumed facts or resultant 



1 Le Vol. Plane Des Oiseaux par G. F. Tydeman. Archives Neerlandaises des 

 Sciences Exactes et Naturelles PubUees par La Societe Hollandaise Des Sciences 

 a Harlem. Series III B (Sciences naturelles), Tome I, l'^ et 2'" Livraisons. La 

 Haye, 1911. 



