234 



CENTRE OF GRAVITY. 



other. If there be three points of support, which are not ranged in the same 

 straight line, the body will be supported in the same manner as it would be by 

 a base coinciding with the triangle formed by straight lines joining the three 

 points of support. In the same manner, whatever be the number of points on 

 which the body may rest, its virtual base will be found by supposiug straight 

 lines drawn, joining the several points successively. When the line of direc- 

 tion falls within this base, the body. will always stand firm, and otherwise not. 

 The degree of stability is determined in the same manner as if the base were 

 a continued surface. 



Necessity and experience teach an animal to adapt its postures and motions 

 to the position of the centre of gravity of his body. When a man stands, the 

 line of direction of his weight must fall within the base formed by his feet. If 

 A B C D, fig. 26, be the feet, this base is the space A B C D. It is evident 



that the more his toes are turned outward, the more contracted the base will 

 be in the direction E F, and the more liable he will be to fall backward or for- 

 ward. Also the closer his feet are together, the more contracted the base 

 will be in the direction G H, and the more liable he will be to fall toward 

 either side. 



When a man walks, the legs are alternately lifted from the ground, and the 

 centre of gravity is either unsupported or thrown from the one side to the other. 

 The body is also thrown a little forward, in order that the tendency of the 

 centre of gravity to fall in the direction of the toes may assist the muscular ac- 

 tion in propelling the body. This forward inclination of the body increases 

 with the speed of the motion. 



But for the flexibility of the knee-joint, the labor of walking would be much 

 greater than it is ; for the centre of gravity would be more elevated by each 

 step. The line of motion of the centre of gravity in walking is represented by 

 fig. 27, and deviates but little from a regular horizontal line, so that the eleva- 



Fig. 27. 



tion of the centre of gravity is subject to very slight variation. But if there 

 were no knee-joint, as when a man has wooden legs, the centre of gravity 



