110 



ANIMAL LOCOMOTIOI^T. 



causing weights acting over pulleys to operate upon the root 

 of the wing {a or e), and watching how far the weights {k or /) 

 are raised. In these calculations allowance is of course to be 

 made for friction. The object of the two sets of experiments 

 described and figured, is to show that the wing can exert a 

 tractile power either in a nearly horizontal direction or in a 

 nearly vertical one, flight being produced in both cases. I 

 wish now to show that a body not supplied with wings or 

 inclined surfaces will, if left to itself, fall vertically down- 

 wards ; whereas, if it be furnished with wings, its vertical fall 

 is converted into oblique downward flight. These are very 

 interesting points. Experiment has shown me that a wing 

 when made to vibrate vertically produces horizontal traction ; 

 when made to vibrate horizontally, vertical traction; the 

 vertical fall of a body armed with wings producing oblique 

 tiaction. The descent of weights can also be made to propel 

 the wings either in a vertical or horizontal direction; the 

 vibration of the wings upon the air in natural flight causing 

 the weights (body of flying creature) to move forward. 

 This shows the very important part performed by weight in 

 all kinds of flight. 



JVeiglit necessary to Flight. — However paradoxical it may 

 seem, a certain amount of weight is indispensable in flight. 



In the first place, it gives peculiar eflicacy and energy to 

 the up stroke, by acting upon the inclined planes formed 

 by the wings in the direction of the plane of progression. 

 The power and the weight may thus be said to reciprocate, 

 the two sitting, as it were, side by side, and blending their 

 peculiar influences to produce a common result. 



Secondly, it adds momentum, — a heavy body, when once 

 fairly under weigh, meeting with little resistance from the 

 air, through which it sweeps like a heavy pendulum. 



Thirdly, the mere act of rotating the wings on and off 

 the wind during extension and flexion, with a slight down- 

 word stroke, apparently represents the entire exertion on the 

 part of the volant animal, the rest being performed by weight 

 alone. 



This last circumstance is deserving of attention, the more 

 especially as it seems to constitute the principal diff'erence 



