152 



ANIMAL LOCOMOTION". 



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 range 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. This is particularly 

 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 percentage 

 of the moving or driving power is inevitably consumed in 

 slip, from the fact of the blades of the screw operating on 

 nearly the same particles of matter. The wings, on the other 

 hand, do not follow each other, but have a distinct recipro- 

 cating motion, i.e. they dart first in one direction, and then 

 in another and opposite direction, in such a manner that they 

 make during the one stroke the current on which they rise 

 and progress the next. The blades formed by the wings 

 and the blur or impression produced on the eye by the blades 

 when made to vibrate rapidly are widely separated, — the one 

 blade and its blur 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 right wing traverses 

 and completely occupies the right half of a circle, and com- 

 presses all the air contained within this space ; the left 

 wing occupying and working up all the air in the left and 

 remaining half The range or sweep of the two wings, when 

 urged to their extreme limits, corresponds as nearly as may 

 be to one entire circle^ (fig. 56, p. 120). By separating 

 the blades of the screw, and causing them to reciprocate, 

 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 



1 Of this circle, the thorax may be regarded as forming the centre, the 

 abdomen, which is always heavier than the head, tilting the body slightly in 

 an upward direction. This tiltirg of the trunk favours flight by causing the 

 body to act after the manner of a kite. 



