Manchester Memoirs, Vol. liii. (1909), No. 15. 3 



wards at, say, 15 miles an hour — between force 2 and 3 

 of Fitzroy's scale : a moderate estimate for autumn. The 

 bird has thus to make its journey in a supporting medium 

 that is moving at the same time in an opposite direction. 

 In one hour it will have flown 25 miles through that body 

 of air, but during the same time the aerial mass has moved 

 15 miles eastwards. Between two points on the earth's 

 surface the bird will have travelled no more than 10 miles. 

 At this rate the journey will take 30 hours. It should be 

 quite evident that no bird can make any headway against 

 a wind with a greater velocity than its own speed of flight. 

 I am unable to refer to any definite statement by one 

 of those who hold this head or beam wind theory, but the 

 general opinion seems to be that when a bird flies head to 

 wind the air rushes past it at a total velocity which is that 

 of the wind plus the velocity of the bird. Of course, this 

 would be quite true in the case of a steamer, a cyclist, a 

 pedestrian, or even a swimming bird. But it is absurd to 

 assume that it is so in the case of a flying bird, or even — 

 the parallel may be useful to some minds — in the case of 

 a swimmer in a strongly flowing current ; the bird and 

 the swimmer are wholly in a moving supporting medium. 

 (I might point out here that it is not safe to study avian 

 flight by the aid of sailing ships. A ship is supported by 

 the water but propelled by the air. The bird is supported 

 by the air and propelled by its wings.) Mr. Headley's 

 remark that the wind " impinges on their backs and so 

 tends to drive them downwards " I fail to understand. 

 The same author, in his "Structure and Life of Birds" 

 (p. 185), compares the flying bird to a skater on thin ice, 

 passing rapidly from one portion of supporting medium 

 to another, none of which is sufficient to support it if the 

 bird or the skater remained stationary. This simile is 

 not, I think, justifiable. 



