114 



ANIMAL LOCOMOTION. 



it is once fairly launched in space and travelling rapidly in a 

 downward direction. 



Weight, Momentum, and Power, to a certain extent, synonymous 

 in Flight. — When a bird rises it has little or no momentum, so 

 that if it comes in contact with a solid resisting surface it 

 does not injure itself. When, however, it has acquired all 

 the momentum of which it is capable, and is in full and rapid 

 flight, such contact results in destruction. My friend Mr. A. 

 D. Bartlett informed me of an instance where a wild duck 

 terminated its career by coming violently in contact with one 

 of the glasses of the Eddystone Lighthouse. The glass, v/hich 

 was fully an inch in thickness, was completely smashed. 

 Advantage is taken of this circumstance in killing sea-birds, 

 a bait being placed on a board and set afloat with a view to 

 breaking the neck of the bird when it stoops to seize the car- 

 rion. The additional power due to momentum in heavy 

 bodies in motion is well illustrated in the start and progress 

 of steamboats. In these the slip, as it is technically called, 

 decreases as the speed of the vessel increases ; the strength of 

 a man, if applied by a hawser attached to the stern, of a 

 moderate-sized vessel, being sufficient to retard, and, in some 

 instances prevent, its starting. In such a case the power of the 

 engine is almost entirely devoted to " slip " or in giving motion 

 to the fluid in which the screw or paddle is immersed. It is 

 consequently not the power residing in the paddle or screw 

 which is cumulative, but the momentum inhering in the mass. 

 In the bird, the momentum, alias weight, is made to act upon 

 the inclined planes formed by the wings, these adroitly con- 

 verting it into sustaining and propelling power. It is to this 

 circumstance, more than any other, that the prolonged flight 

 of birds is mainly due, the inertia or dead weight of the 

 trunk aiding and abetting the action of the wings, and so 

 relieving the excess of exertion which would necessarily 

 devolve on the bird. It is thus that the power which in 

 living structures resides in the mass is conserved, and the 

 mass itself turned to account. But for this reciprocity, no 

 bird could retain its position in the air for more than a few 

 minutes at a time. This is proved by the comparatively 

 brief upward flight of the lark and the*hovering of the hawk 



