202 



MONTHLY JOURNAL OF AGRICULTURE. 



ty is either unsupported or thro-vvn from 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 action 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 elevation of the cen- 



Fig. 27. 



tre of gravity is subject to very slight variation. 

 But if there were no knee-joiut, as when a man 

 has woodfen legs, the centre of gravity would 

 move as in fig. 28, so that at each step the weight 



of the body would be lifted through a consider- 

 able bight, and therefore the labor of walking 

 would be much increased. 



If a man stand on one leg, the line of direc- 



Fig. 28. 



tion of his weight must fall within the space on 

 which his foot treads. The smallness of this 

 space, compared with the hight of the centre 

 of gravit\', accounts for the difficulty of this feat. 

 The position of the centre of gravity of the 

 body changes with the posture and position of 

 the limbs. If the arm be extended from one 

 side, the centre of gravity is brought nearer to 

 that side than it was when the arm hung per- 

 pendicularly. ^Vhen dancers, standing on one 

 leg, extend the other at right angles to it, they 

 must incline the body in the direction opposite 

 to that in which the leg is extended, in order to 

 bring the centre of gravity over the foot which 

 supports them. 



When a porter carries a 

 load, his position must be 

 regulated bj- the centre of 

 fjravity of his bodj- and the 

 load taken toarether. If he 

 bore the load on his back, 

 fig. 29, the line of direction 

 would pass beyond his 

 heels, and he would fall 

 backward. To bring the 

 centre of gravity over his 

 feet, he accordingly leans 

 foi-ward, fig. 30. 



If a nurse carrj^ a child 

 in her arms, .she leans back 

 for a like rerison. 



When a load is carried 

 on the head, the bearer 



Fig. 29. 



stands upright, that the can- Fig. 30. 



tre of gravity may be over 

 his feet. In ascending a 

 hill, we appear to incline for- 

 w-ard, and in descending, to 

 lean backward ; but in truth 

 we are standing upright 

 with respect to a level 

 plane. Tliis is nece.ssary 

 to keep the line of direction 

 between the feet, as is evi- 

 dent from fig. 31. 



A person sitting on a 

 chair which has no back, 

 cannot rise fi-om it without 

 eitlier stooping foi-ward to 

 bring the centre of gravity 

 over the feet, or drawing 

 back the feet to bring them 

 under the centre of gravity. 



A quadruped never raises both feet on the 

 same side simultaneously,for the centre of grav- 

 ity would then be unsupported. Let A B C D, 

 fig. 32, be the feet. The base on which it stands 

 is A B CD, and the centre of gravity is nearly 

 over the point O, where the diagonals cro.s3 

 each other. The legs A and C being raised to- 

 gether, the centre of gravity is supported by the 

 legs B and D, since it falls between them ; and 

 when B and D are raised, it is, in like manner, 

 supported by the feet A and C. The centre of 

 gravity, hovirever, is often un.supported for a 

 moment ; for the leg B is rai.sed from the ground 



