Fig. 112, p. 217, embodies M. de la Landelle’s ideas. [105] Report on the First Exhibition of the Aëronautical Society of Great Britain, held at the Crystal Palace, London, in June 1868, p. 10. [106] Mons. Nadar, in a paper written in 1863, enters very fully into the subject of artificial flight, as performed by the aid of the screw. Liberal extracts are given from Nadar’s paper in Astra Castra, by Captain Hatton Turner. London, 1865, p. 340. To Turner’s handsome volume the reader is referred for much curious and interesting information on the subject of Aërostation. [Illustration: FIG. 112.--Flying Machine designed by M. de la Landelle.] In the helicopteric models made by MM. Nadar, Pontin d’Amécourt, and de la Landelle, the screws (_m n o p q r s t_ of figure) are arranged in tiers, _i.e._ the one screw is placed above the other. In this respect they resemble the aëroplanes recommended by Mr. Wenham, and tested by Mr. Stringfellow (compare _m n o p q r s t_ of fig. 112, with _a b c_ of 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 aërial 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 sustaining 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 system 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 forming, and to avoid natural currents when not adapted for propelling or sustaining purposes (fig. 67, 68, 69, and 70, p. 141).