18 SCIENCE OF COMMON THINGS. 



Eope-dancing. How we learn to walk. 



100 What is the reason that persons walking arm-in-arm shake and 

 jostle each other, unless they make the movements of their feet to correspond, 

 as soldiers do in marching ? 



When we walk at a moderate rate, the centre of 

 gravity comes alternately over the right and over the 

 left foot. The body advances, therefore, in a waving 

 line; and unless two persons walking together keep 

 step, the waving motion of the two fails to coincide. 



101 Where would the centre of gravity be in a wheel made entirely of 

 wood and of a uniform thickness ? 



In the centre. 



1OS Where would the centre of gravity be if a part of the rim of the 

 same wJieel were made of iron f 



It would be changed to some point aside from the 

 centre of the wheel. 



103 In what does the art of balancing or walking upon a rope consist? 



In keeping the centre of gravity in a line over the 

 base upon which the body rests. 



104 What is the base upon which the human body rests or is supported t 



The two feet and the space included between them. 



105 Why is it a very difficult thing for children to learn to walk ? 



In consequence of the natural upright position of the 

 human body, it is constantly necessary to employ some 

 exertion to keep our balance, or to prevent ourselves 

 from falling, when we place one foot before the other. 

 Children, after they acquire strength to stand, are 

 obliged to acquire this knowledge of preserving the 

 balance by experience. When the art is once acquired, 

 the necessary actions are performed involuntarily. 



106 Why do young quadrupeds learn to walk much sooner than chil- 

 dren f 



Because a body is tottering in proportion to its great 

 altitude and narrow base. A child has a body thus 

 constituted, and learns to walk but slowly because of 

 this difficulty, (perhaps in ten or twelve months,) while 

 the young of quadrupeds, having a broad supporting 

 base, are able to stand and move about almost immedi- 

 ately. 



107 Are aU the limbs of a tall tree arranged in such a manner, that tlie 



