PHYSIOLOGY OP MOVEMENT. 755 



gait. In this act the centre of gravity is not moved directly forward, 

 but oscillates from side to side. 



(e) Swimming. — In swimming the trunk is almost entirely immersed 

 in the water, the neck extended, and the head held high. The horse 

 swims readily, since its lungs contain a large amount of air, and the rapid 

 movement of the limbs gives to the body an impulse in a horizontal 

 direction, the weight of the body being largely overcome by the buoy- 

 ancy of the water. The movement of the feet consists in rapid pen- 

 dulum-like motions from before backward, and the forward motion of 

 the body results from the resistance which the water offers to the back- 

 ward movement of the feet. The swimming motions in the horse occur 

 regularly in the same direction on each side of the body. 



The power developed by any mechanical contrivance is estimated 

 by the weight multiplied by the height to which it may be raised in one 

 second; this is described as a kilogram meter. Animals, therefore, may 

 be regarded as machines, since they possess the power of raising a weight 

 to a certain height. Likewise, the motion of a weight on a level surface 

 is to be regarded as a production of work, and whose movement equals 

 the sum of all the resistances which have to be overcome by the animal 

 and the velocity produced. The power of a horse is placed, as an average, 

 at sixty kilogram meters, an ox at sixty kilogram meters, and an ass at 

 thirty-six kilogram meters. The average velocity communicated to the 

 weight in work continued for several hours each day has been placed in 

 the horse at 1.25 meters, the ox 0.8 meter, and the ass 0.8 ; consequently 

 the unit of power in the horse has been placed at seventy-five kilogram 

 meters, in the ox forty-seven, and ass twenty -nine: so that, therefore, one 

 horse-power would mean a power which would be able to raise seventy- 

 five kilograms one meter high in one second. In referring these figures 

 to the body of animals, it has been calculated that the development of 

 power in animals in one hour, reckoned for each kilogram of body 

 weight, is — 



In the horse, ..... 940 kilogram meters. 



" " mule, .... 800 



" " ass 640 



" " ox, 620 



" " man, . . . 560 



In the application of animal power in hauling, or in employing the 

 horse as a draught animal, resistance has to be overcome, since the centre 

 of gravity must be advanced by the power exerted by the hind extremi- 

 ties (Pig. 312). In general, the hauling power of an animal, as in all 

 motors, is governed by the mass moved, therefore through the weight 

 of the animal and through the velocity communicated. The power thus 

 acts in two relationships, through the overcoming of the resistance of the 



