442 Prof. S. P. Langley on tJie 



steady wind the rate of rise diminishes, the more rapid the 

 pulsations, the nearer the mean rate of rise will he to the 

 initial rate. The requisite frequency of pulsations is also 

 related to the inertia of the plane, as the less the inertia, the 

 more frequent must he the pulsations in order that the plane 

 shall not lose its relative velocity. 



It is obvious that there is a limit or weight which cannot 

 be exceeded, if the body is to be sustained by any such fluctua- 

 tions of velocity as can be actually experienced. Above this 

 limit of weight the body will sink. Below this limit the 

 lighter the body is the higher it will be carried, but with 

 increasing variability of speed. That body, then, which has 

 the greatest weight per unit of surface will soar with the 

 greatest steadiness, if it soar at all, not on account of this 

 weight per se, but because the weight is an index of its 

 inertia. 



The reader who will compare the results of experiments 

 made with any artificial flying models, like those of Penaud, 

 with the weights of the soaring birds as given in the tables by 

 M. Mouillard, or other authentic sources, cannot fail to be 

 struck with the great weight in proportion to wing- surface 

 which nature has given to the soaring bird, compared with 

 any which man has yet been able to imitate in his models. 



This fact of the weight of the soaring bird in proportion to 

 its area has been again and again noted, and it has been fre- 

 quently remarked that without weight the bird could not 

 soar, by writers who felt that they could very safely make 

 such a paradoxical statement, in view of the evidence nature 

 everywhere gave that this weight was indeed in some way 

 necessary to rising. But these writers have not shown, so 

 far as I remember, how this necessity arises, and this is what 

 I now endeavour to point out*. 



It has not here been shown what limit of weight is im- 

 posed to the power of an ordinary wind to elevate and sus- 

 tain, but it seems to me, and I hope that it may seem to the 

 reader, that the evidence that there is some weight which the 

 action of the wind is sufficient to permanently sustain under 

 these conditions in a free body, has a demonstrative character, 

 although no quantitative formula is offered at this stage of 

 the investigation. It is obvious that, if this weight is sus- 



* It is perhaps not superfluous to recall here that, according to the 

 researches of Rankine, Froude, and others, a body moulded in wave-line 

 curves would, if frictionless, continue to move indefinitely against an 

 opposed wind, in virtue of inertia and once acquired velocity, and also to 

 recall how very small the effect of fluid friction in the air has been shown 

 to be (by the writer in a previous investigation). 



