ANIMAL MECHANICS. FORMS OF PROGRESSION. 167 



in that fluid. Accordingly, a very small proportion of the muscular 

 power exerted is absorbed in the effort to sustain, or lift, the centre of 

 gravity ; whilst by far the larger proportion of that force can be em- 

 ployed in the progressive movement. The mode in which that force 

 is exerted upon the water behind the animal, so as to overcome the 

 resistance of that fluid in front of it, will be explained in speaking of 

 the swimming of fish. 



In flight, or progression through the air, the resistance of the me- 

 dium in which the animal moves, is reduced to its lowest possible degree, 

 air being proportionally so light and elastic. But the difficulty of 

 moving in it is thereby greatly increased ; for almost the entire weight 

 of the animal's body has to be raised from the earth, against its gravity, 

 only so much of that weight being supported by the air, as is equal to 

 the weight of an equal bulk of air, i. e. an almost inappreciable quan- 

 tity. In this kind of movement, therefore, the principal object to be 

 attained by a bird, for example, is to lift and sustain its weight in the 

 air ; the progressive movement forward, being the result of the co-op- 

 eration of the gravity of the animal acting downwards, and the lifting 

 power acting upwards and forwards. The feeble resistance of the air 

 to the movement of the bird, is more than counterbalanced by its im- 

 perfect character as a medium for support, and for the development of 

 that reaction which is necessary for the ascensive and progressive 

 movements of an animal in it. The mariner in which, by special con- 

 trivances, this reactionary force is obtained, will be described under 

 the head of flight in birds. 



On comparing the three chief modes of progression, it appears that 

 less force is needed in the fish moving in water, than in the quadruped 

 moving over land, and less in the latter than in the bird moving 

 through the air. Nevertheless, it is a good illustration of the perfection 

 of the mechanical and physiological adaptations of animals, to find that 

 the velocity attainable by certain birds is greater than that attained 

 by any quadruped or fish; the rate of movement of the hawk is said 

 to be 150 miles per hour ; that of the swiftest race-horse, Eclipse, 56 

 miles per hour ; and that of the salmon 20 or 25 miles per hour. The 

 quick walking pace of man is about 5 miles, and his running pace 

 about 10 miles per hour. 



Locomotion of Man on Solids. 



The only position of the human body, in which the muscles are en- 

 tirely passive and relaxed, is the recumbent posture ; the respiratory 

 muscles in that case being alone necessarily in action. In raising the 

 body from such a position, the lower limbs are usually drawn up ; the 

 heels are planted on the supporting surface, to offer a steady basis of 

 support ; one or both upper limbs are put forward, to assist in balanc- 

 ing the trunk upon the tuberosities of the ischial bones, an act which 

 is partly performed by the muscles which pass from the thigh to the 

 pelvis and vertebral column ; the vertebral column itself is maintained 

 in position by the powerful muscles of the abdomen, acting from the 



