PETTIGREW ON THE MECHANISM OF FLIGHT. 229 



2. The vanes of the ordinary two-bladed screw have, as a rule, a very limited area, the 

 area corresponding to their area of revolution. The wings, on the other hand, have a 

 vast range (Plate XV. figs. 58, 59, and 61), and during their elevation and depression 

 rush through an extensive space, the slightest movement at the root or axis of the wing 

 being followed by a gigantic up or down stroke at the other. As a consequence, the 

 wings as a rule act upon successive and undisturbed strata of air. The advantage gained 

 by this arrangement in a thin medium like the air, where the quantity to be compressed 

 is necessarily great, is simply incalculable. 



3. In the ordinary screw the blades follow each other in rapid succession, so that they 

 travel over nearly the same space, and operate upon nearly the same particles (whether 

 water or air), in nearly the same interval of time. The limited area at their disposal is 

 consequently not utilized, the action of the two blades being confined, as it were, to the 

 same plane, and the blades being made to precede or follow each other in such a manner 

 as necessitates the work being virtually performed only by one of them. Tl i is is particul; i rly 

 the case when the motion of the screw is rapid and the mass propelled is in the act of 

 being set in motion, i. e. before it has acquired momentum. In this instance a large per- 

 centage of the moving or driving power is inevitably consumed in slip, from the feet of t he 

 blades of the screw operating on nearly the same particles of matter. It is otherwise with 

 the wings. Here the blades, and the blurs which the blades produce when moving, are 

 widely separated, — the one being situated on the right side of the body and corresponding 

 to the right wing, the other on the left and corresponding to the left wing. The wings do 



not therefore follow each other and travel over the same space, or compress the same 

 particles, at any stage of their progress. On the contrary, the right wing traverses and 

 completely monopolizes the right half or hemisphere of a circle, the left wing in like 

 manner appropriating and converting to its own uses the left and remaining half (Plate 

 XV. figs. 58 and 59). The range or sweep of the two wings, when urged to their ex- 

 treme limits, corresponds as nearly as may be to one entire circle *. By thus separating 

 the blades of the screw, as happens in the wings, a double result is produced, since the 

 blades always act upon independent columns of air and in no instance overlap or 

 double upon each other. The advantages possessed by this arrangement are particularly 

 evident when the motion is rapid— the natural screws formed by the wings being most 

 efficient when the artificial ones are least so. As there seems to be no limit to the velocity 

 with which the' wings may be driven, and as increased velocity necessarily results in 

 increased elevating, propelling, and sustaining power, we have here a striking example 

 of the manner in which Nature triumphs over art even in her most ingenious, skilful, and 



successful creations. 



There is yet another advantage which ought not to be overlooked. The same power 

 which propels a screw composed of two blades will suffice, or very nearly suffice, for 

 driving the detached widely separated blades of the screw formed by the wings. 



4. The vanes or blades of the screw, as commonly constructed, are fixed at a given 



• * Of this circle the thorax may he regarded as forming the centre, the abdomen, which is always heavier than the 

 head, tilting the body slightly in an upward direction. This tilting of the trunk favours flight by causing the body to 



act after the manner of a kite. 

 TOL. XXVI. 



2 



