S. P. Langley — Internal Work of the Wind. 57 



It follows from the diagram (fig. 1) that other things being 

 equal the more frequent the wind's pulsations, the greater will 

 be the rise of the plane, for since during each period of steady 

 wind, the rate of rise diminishes, the more rapid the pulsa- 

 tions, the nearer the mean rate of rise will be 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 fre- 

 quent must be 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 every- 

 where gave that this weight was indeed in some way neces- 

 sary to rising. But these writers have not shown, so far as I 

 remember, how this necessity arises, and this is what I now 

 endeavor 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 sustainable 



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

 of Rankine, Froude and others, a body moulded m 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 investi- 

 gation). 



