746 THE POPULAR SCIENCE MONTHLY. 



the lightness and the strength of the bones, all show that every- 

 thing is subordinated to this one supreme function. In compar- 

 ing a bird with an artificial flying machine it is necessary to bear 

 this in mind. 



But it is in the use of the wing as an aeroplane that the most 

 wonderful feats of bird locomotion consist. If we are ever to 

 achieve artificial flight it must be by the application of the prin- 

 ciples underlying these. There are four of these feats of bird 

 flight which require special notice as bearing on the subject of 

 artificial flight. These are hovering, poising, soaring, and sailing. 



Hovering. There is some confusion in the use of this term. 

 It always refers to a maintenance of a body in one position in the 

 air ; but this may be done in two ways either by vigorous flap- 

 ping of the wings, or else, under certain conditions, with no mo- 

 tion of the wings at all. This latter, however, I shall call poising, 

 and confine the term hovering to the former. In this sense hov- 

 ering is seen in many insects and in the humming bird, and, 

 among larger birds, in the sparrow hawk {Falco sparverius) and 

 in the osprey {Pandion halicetus). In these it is seen that in hov- 

 ering the body is inclined upward, and the stroke of the wing is 

 decidedly forward as well as downward. The reason of this is, as 

 already explained, that downward strokes give onward motion. 

 But tlie main thing to be observed in large birds is the violent 

 struggles necessary to maintain position compared with the ease 

 of onward flight. This difference furnishes the key to the prop- 

 erties of an aeroplane, and was, I believe, first explained by 

 Marey. In maintaining the body in the same position, as in hov- 

 ering, the air gives way under each stroke of the wing, creating 

 a downward current, thus greatly diminishing the effectiveness 

 of the downstroke and increasing the loss in recovery or up- 

 stroke. In progressive flight, on the contrary, and more and more 

 as the progress is more rapid, every phase of the downstroke is on 

 new air. The wing strikes on three feet or six feet or ten feet or 

 twenty feet of air, according to the velocity of progress, with 

 every stroke. The air has not time to give way before the wing 

 passes on to new, unmoved air. But if it is difficult to maintain 

 one position, as in hovering, it is evidently still more difiicult, on 

 this principle, to raise the body directly upward. This explains 

 the difficulty experienced by a large bird like the condor in rising, 

 and yet the ease and grace of progressive flight when well up. 

 We will see hereafter the great importance of this principle, as 

 shown by the experiments of Langley. 



Poising. By this term I mean the maintenance of a fixed 

 position with outstretched, motionless wings. During my boy- 

 hood I was fond of field sports of all kinds, and therefore a con- 

 stant and accurate observer of the flight of birds ; and yet, dur- 



