218 



AERONAUTICS. 



exceedingly fragile, and because of the prodigious force 

 required to propel them usually break after a few trials. 

 Fig. 112, p. 217, embodies M. de la Landelle's ideas. 



In the helicopteric models made by MM. Nadar, Pontin 

 d'Amecourt, and de la Landelle, the screws (mnopqrst of 

 figure) are arranged in tiers, ie. the one screw is placed 

 above the other. In this respect they resemble the aero- 

 planes recommended by Mr. Wenham, and tested by Mr. 

 Stringfellow (compare mnopqrst of fig. 112, with ah c oi 

 fig. 110, p. 213). The superimposed screws, as already 

 explained, were first figured and described by Sir George 

 Cayley (p. 215). The French screws, and that employed by 

 Mr. Phillips, are rigid or unyielding, and strike the air at a 

 given angle, and herein, I believe, consists their principal 

 defect. This arrangement results in a ruinous expenditure of 

 power, and is accompanied by a great amount of slip. The 

 aerial screw, and the machine to be elevated by it, can be set 

 in motion without any preliminary run, and in this respect it 

 has the advantage over the machine supported by mere sus- 

 taining planes. It has, in fact, a certain amount of inherent 

 motion, its screws revolving, and supplying it with active or 

 moving surfaces. It is accordingly more independent than 

 the machine designed by Henson, Wenham, and Stringfellow. 



I may observe with regard to the systein of rigid inclined 

 planes wedged forward at a given angle in a straight line or 

 in a circle, that it does not embody the principle carried out 

 in nature. 



The wing of a flying creature, as I have taken pains to 

 show, is not rigid ; neither does it always strike the air at 

 a given angle. On the contrary, it is capable of moving in 

 all its parts, and attacks the air at an infinite variety of 

 angles (pp. 151 to 154). Above all, the surface exposed by 

 a natural wing, when compared with the great weight 

 it is capable of elevating, is remarkably small (fig. 89, 

 p. 171). This is accounted for by the length and the great 

 range of motion of natural wings; the latter enabling the 

 wings to convert large tracts of air into supporting areas (figs. 

 64, 65, and 66, p. 139). It is also accounted for by the 

 multiplicity of the movements of natural wings, these enabling 

 the pinions to create and rise upon currents of their own 



