578 PHILOSOPHICAL TRANSACTIONS. [aNNO 1682-3. 



that the retention of a joint stretched out is not from the tonical action of an- 

 tagonist muscles. 



Hence he goes on to deliver the various postures of an animal, which he does 

 by assigning his centre of gravity in all his possible positions. As in a man 

 stretched out at length, the centre is between the nates and pubes. That a 

 man cannot well stand on one heel, or the tip of a toe, because in these cases 

 the line of direction falls without his basis, &c. 



That though birds have 1 feet, yet they neither walk nor stand the same way 

 as a man; which depends on the different structureof their joints. Forj 1, they 

 differ in the number of the bones. 2. In the form. 3. In the distribution and 

 make of their muscles. 4. In the joints themselves. 



He demonstrates the manner how a bird when sleeping sits firm on a twig, 

 though the muscles are then inactive; namely, by a strong constriction of its 

 claws, and consequently a firm comprehension of that twig, necessarily and 

 mechanically resulting from the gravity of the bird, and the shortness of the 

 tendons of those muscles that contract the claws. 



That quadrupeds cannot stand in their natural prone position on 1 or 2 feet, 

 because the centre of gravity and its line of propension cannot fall in either, or 

 between both. 



He shows the art of seating upon ice, as also how progression in quadrupeds 

 is performed, and likewise leaping, in which the vis motiva is to the weight of 

 the body as 2,900 is to 1 . That in leaping according to a line inclined to the 

 horizon, at oblique angles, the line described by the centre of gravity shall be 

 a curve parabola, as being compounded of the straight uniform motion forward, 

 and the accelerated descent of the heavy body. Next he gives an exact account 

 of flying, the main stress of which is in the largeness of the muscles that move 

 the wings, the potentia of which exceeds the weight of the bird 10,000 times; 

 with many more curious particulars about their several ways of flying. 



He describes the action of swimming, and how fishes change their specified 

 gravity on occasion, by the compression and dilatation of the air contained in 

 their air-bladders performed by the many and strong muscles about their bellies. 

 He assigns the reason why man does not swim by instinct as well as other ani- 

 mals, to be chiefly on account of the gravity of the head so much exceeding 

 the proportion of that of the rest of the body. 



The several ways to live and move under water were described before, as the 

 diving bell, the leathern cylinder, &c. but that which he seems most to insist 

 on, is of a brass or copper vesica, about 2 feet diameter, to contain the diver's 

 head, this to be fastened to a goat's-skin habit fitted exactly to the shape of the 

 body. He contrives a circulation for the air by pipes within the vesica, and 



