440 DR PETTIGKEW ON THE PHYSIOLOGY OF WINGS. 



flexible and elastic, it accommodates itself to the speed at which it is driven, and 

 gives a uniform buoyancy. The slip I may add is nominal in amount. This 

 screw is exceedingly light, and owes its efficacy to its shape and the graduated 

 nature of its blades, the anterior margin of each blade being comparatively 

 rigid, the posterior margin being comparatively flexible and more or less elastic. 

 The blades are kites in the same sense that natural wings are kites, and are 

 flown as such when the screw revolves. I find the aerial wave screw flies best 

 and elevates most when its blades are inclined at a certain upward angle as 

 indicated in the figure (73). The aerial wave screw may have the numbers of 

 its blades increased by placing the one above the other, and two or more screws 

 may be combined and made to revolve in opposite directions so as to make 

 them reciprocate, the one screw producing the current on which the other 

 rises, as happens in natural wings. 



Fig. 73.* 



The Aerial Wave Screw operates also upon Water. — The form of screw just 

 described is adapted in a marked manner for water, if the blades be made of 

 carefully tempered finely graduated steel plates and reduced in size. It bears 

 the same relation to, and produces the same results upon, water as the tail and 

 fin of the fish. It throws its blades during its action into double figure of 8 

 curves, similar in all respects to those produced on the anterior and posterior 

 margins of the natural and artificial flying wing. As the speed attained by the 

 several portions of each blade varies, so the angle at which each part of the 



* Fig. 72. Aerial wave screw whose blades are slightly twisted upon themselves (a b, c d; c f, g h), so that those 

 portions nearest the root (d h) make a greater angle with the horizon than those parts nearer the tip {bf). The angle 

 is thus adjusted to the speed attained by the different portions of the screw. The angle admits of further adjustment 

 by means of the steel springs z, s, these exercising a restraining, and to a certain extent a regulating influence which 

 effectually prevents shock. 



It will be at once perceived from this figure that the portions of the screw marked m and n travel at a much lower 

 speed than those portions marked o and p, and these again more slowly than those marked q and r. As however the 

 angle which a wing or a portion of a wing, as I have pointed out, varies to accommodate itself to the speed attained by 

 the wing, or a portion thereof, it follows, that to make the wave screw mechanically perfect, the angles made by its 

 several portions must be accurately adapted to the travel of its several parts as indicated above. 



x, Vertical tube for receiving driving shaft, v, w, Sockets in which the roots of the blades of the screw rotate, 

 the degree of rotation being limited by steel springs z, s. a b, e f, Tapering elastic reeds forming anterior or thick 

 margins of blades of screw, d c, h g, Posterior or thin elastic margins of blades of screw, m n, o ]>, q r, Radii formed 

 by the different portions of the blades of the screw when in operation. The arrows indicate the direction of travel. 



