pable of rising to a certain height ; I have uniformly observed, that in 

 the most celebrated public dancers, the trunk, and especially the lower 

 limbs, are very muscular, the calf of the leg, the buttocks, and the bkck 

 indicate, by their bulk, a remarkable degree of energy in the extensors, 

 by whose action, leaping is chiefly effected. 



A dancer who rises vertically, falls back to the ground, when the Force 

 of gravitation exceeds the impulse which he had received ; his fall re- 

 sembles that of a projectile in vertical motion ; it takes place, according 

 to a descending line that is perfectly similar, in direction and height to 

 the ascending line. 



The same thing takes place in the oblique leap, except, however, that 

 the body, like a shell projected by the explosion of gunpowder, describes 

 a parabolic curve, ascending as long as the impelling power exceeds the 

 force of gravitation : descending, when the latter, which increases during 

 the progress of the leap, is equal to the force of impulse. This takes 

 place when the body has described a curve which represents the half of 

 a parabola ; from that moment, the force of gravitation goes on increas- 

 ing, and the body descends in a curve corresponding to the first*. 



CXC. Of Swimming. Few animals have more difficulty than man, in 

 supporting themselves on the surface of a fluid ; yet the weight of the hu- 

 man body exceeds but little that of the same bulk of water : sometimes 

 even, when the body is loaded with much fat, its specific gravity and that 

 of water are the same. Hence, it is observed, that corpulent men swim 

 with less effort ; but the weight is not equally distributed over every 

 point of the supporting fluid. The head, whose relative weight is very 

 con sderable, is the principal difficulty in swimming, and it requires some 

 effort to keep it. raised, so as to allow the air to enter freely into the lungs, 

 through the mouth and nostrils. The upper and lower limbs act alter- 

 nately against the water which they displace by pressing on it. In these 

 various motions, fhere is a successive flexion, extension, abduction, and 

 adduction of the limbs ; most of the muscles of the body are in motion, 

 and have their fixed point of action in the chest, which swimmers keep 

 expanded by retaining, by a constriction of the glottis, a considerable 

 quantity of air within the pulmonary tissue. This continuous dilatation of 

 the chest is attended with this further advantage, that it renders the bo- 

 dy specificially lighter. The force with which the swimmer is oblig- 

 ed to strike the water, the rapidity with which the motions must succeed 

 each other, that the fluid may yield him a sufficiently fixed point of ac- 

 tion, accounts for the fatigue with which this exertion is attended. 



Fishes are adapted by their structure, to the element in which they 

 live the form of their body, bounded, every where, by salient angles, 

 is well calculated to separate the columns of a fluid. A bladder filled 

 with azote, which is expelled at pleasure, renders their specific gravity 

 less than that of water, according to the quantity of gas it contains ; last- 

 ly, their tail moved by powerful muscles, may be considered as an oar of 

 great strength, the motions of which impel the fish forward, while the 

 fins, like so many secondary oars, facilitate and direct his motions. 



The air bladder of fishes gives to their back a sufficient degree of light- 

 ness to enable it to remain upward, else this part of the body, which is 

 the heaviest, would draw after it the rest, and the animal, lying on his 



* In saltu ad horizontum, obliquo, motusjit perlineamparaboUcamproxime, Borelli. op. 

 cit prop. 178. Vid. Galileo on the motion of projectiles. 



