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 

 forward, fig. 30. 



If a nurse carry a child in her arms, she leans back for a like reason. 



When a load is carried on the head, the bearer stands upright, that the centre 

 of gravity may be over his feet. In ascending a hill, we appear to incline for- 

 ward, and in descending, to lean backward ; but in truth we are standing up- 

 right with respect to a level plane. This is necessary to keep the line of 

 direction between the feet, as is evident from fig. 31. 



Fig. 31. 



A person sitting on a chair which has no back, cannot rise from it without 

 either stooping forward 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 gravity would then be unsupported. Let A B C D, fig. 32, be the 



Fig. 32. 



feet. The base on which it stands is A B C D, and the centre of gravity is 

 nearly over the point O, where the diagonals cross each other. The legs A 

 and C being raised together, 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, however, 



