PKOGKESSION IN OR THEOUGII THE AIR. 



153 



arrangement are particularly evident when the motion is 

 rapid. If the screw employed in navigation be driven beyond 

 a certain speed, it cuts out the water contained within its 

 blades ; the blades and the water revolving as a solid mass. 

 Under these circumstances, the propelling power of the screw 

 is diminished rather than increased. It is quite otherwise 

 witli the screws formed by the wings ; these, because of their 

 reciprocating movements, becoming more and more effective 

 in proportion as the speed is increased. As there seems to 

 be no limit to the velocity with which the wings may be 

 driven, and as increased velocity necessarily results in in- 

 creased 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. 



4. The vanes or blades of the screw, as commonly con- 

 structed, are fixed at a given angle, and consequently always 

 strike at the same degree of obliquity. The speed, moreover, 

 with which the blades are driven, is, as nearly as may be, 

 uniform„ In this arrangement power is lost, the two vanes 

 striking after each other in the same manner, in the same 

 direction, and almost at precisely the same moment, — no 

 provision being made for increasing the angle, and the pro- 

 pelling power, at one stage of the stroke, and reducing it at 

 'another, to diminish the amount of slip incidental to the 

 arrangement. The wings, on the other hand, are driven at a 

 varying speed, and made to attack the air at a great variety 

 of angles j the angles which the pinions make with the hori- 

 zon being gradually increased by the wings being made to 

 rotate on their long axes during the down stroke, to increase the 

 elevating and propeMing power, and gradually decreased during 

 the up stroke, to reduce the resistance occasioned by the wings 

 during their ascent. The latter movement increases the sustain- 

 ing area by placing the wings in a more horizontal position. It 

 follows from this arrangement that every particle of air within 

 the wide range of the wings is separately influenced by them, 

 both during their ascent and descent, — the elevating, propel- 

 ling, and sustaining power being by this means increased to a 

 maximum, while the slip or waftage is reduced to a minimum. 

 8 



