18 THE I.VIKUNAL WORK OF THE WIND. 



pulsations, the Dearer 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 frequent must be the pulsations in order thai the plane shall not 

 lose its relative velocity. 



It is obvious that there is a limit of weight which cannot be exceeded if the 

 body is to be sustained by any such fluctuations of velocity as can be actually ex- 

 perienced. 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 Peuaud, 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 auy 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 frequently 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 

 endeavor to point out.* 



It has not here been shown what limit of weight is imposed t<> the power of an 

 ordinary wind to elevate and sustain, but it seems to me, and I hope that it may so 

 seem to the leader, 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 at any 

 height, gravity may be utilized to cause the body (winch we suppose to be a 

 material plane) to descend on an inclined course, to some distance, even against 

 the wind. 



I de-ire in this connection to remark that the preceding experiments and 



* It is perhaps not superfluous to recall lure 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). 



