52 WELLS'S NATURAL PHILOSOPHY. 



2. Why is a turtle placed on its back unable to move ? 



Because the center of gravit\- of the turtle is, m this position, at the lowest 

 point, and the animal is unable to change it ; therefore it is obliged to remain 



at rest. 



3. Why do very fat people throw back their head and shoulders when they walk ? 



In order that they may eflectually keep the center of gravity of the body 

 over the base formed by the soles of the feet. 



4 Why can not a man, standing with his heels close to a perpendicular wall, bend over 

 sufficiently to pick up any object that lies before him on the ground, without falling? 



Because the wall prevents him from throwing part of his body backward, 

 to counterbalance the head and arms that must project forward. 



5 What is the reason that persons walking arm-in-arra 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 center of gravity comes alternately 

 over the right and over the left foot. The bodj' advances, therefore, in a wav- 

 ing line; and unless two persons walking together keep step, the waving mo- 

 tion of the two fails to coincide. 



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



In keeping the center of gravity in a' line over the base upon which tho 

 body rests. 



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



In consequence of the natural upright position or the human body, it is 

 constantly necessary to employ some exertion to keep our balance, or to pre- 

 vent ourselves from falling, when we place one foot before the other. Chil- 

 dren, after they acquire strength to stand, are obliged to acquire this knowl- 

 edge of preserving the balance by experience. AVhen the art is once ac- 

 quired, the necessary actions are performed involuntarily. 



8. Why do young quadrupeds learn to walk much sooner than children ? 

 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 be- 

 cause 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 immediately 



9. Are all the limbs of a tall tree arranged in such a manner, that the line directed 

 from the center of gravity is caused to fall within the base of the tree ? 



Nature causes the various limbs to shoot out and grow from the sides 

 with as much exactness, in respect of keeping the center of gravity withia 

 the base, as though they had been all arranged artificially. Each limb grows, 

 in respect to all the others, in such a maimer as to preserve a due balance be- 

 tween the whole. 



