40 



MECHANICS. 



A firmly- set fruit-ladder. 



and 30). Another reason is, the large wheel does not 



sink into the smaller cavities in the road. 



A self-supporting frait-ladder (fig. 31) (the centre of 



gravity, when in use, being at or near the top) must have 



its legs more widely 

 ^'s- ^- ^'s- 32. spread, to be secure from 



falling, than if the centre 

 were lower down. Hence 

 such a position, as in fig. 

 32, would be unsafe. 



The support of the 

 human body, in standing 

 and walking, exhibits 

 some interesting exam- 

 ples in relation to this 

 subject. A child can not learn to walk until he acquires 

 skill enough to keep his feet always in the line of direc- 

 tion. When lie fails to do this, he topples over toward 

 the side where the line falls outside his feet. A man stand- 

 ing with his heels touching the wash-board of a room can 

 not possibly stoop over without falling, because, when he 

 bends, the line of direction fiiUs forward 

 of his toes, the wall against which he 

 stands preventing the movement of his 

 body backward to preserve the balance. 



In walking, the centre rises and falls 

 slightly at each step, as shown by the 

 waved line in fig. 33. If it were not 

 for the bending of the knee-joints, this 

 exercise would be much more laborious, 

 as it would then become needful to 

 throw the centre into an upward curve at every step. 

 For this reason, a wooden leg is more imperfect than the 

 natural one (fig. 34). Hence the reason why walking on 

 crutches is laborious and fatiguing, because at every on- 



Fig. 33. 



