SCIENCE OF COMMON THINGS. 



15 



Centre of gravity. 



Position in which a body can rest. 



JVb ; the attraction of the earth for a body varies 

 inversely with the square of its distance from the 'centre. 



86 How can this be illustrated ? 



In the following manner : If one body attracts an- 

 other with a certain force at the distance of one mile, it 

 will attract with four times the force at half a mile, 

 nine times the force at one-third of a mile, and so on in 

 like proportion. On the contrary, it will attract with 

 but one-fourth of the force at two miles, one-ninth of 

 the force at three miles, one-sixteenth of the force at 

 four miles, and so on as the distance increases. 



87 What do we mean by the centre of gravity ? 



That point in a body about which, if supported, the 

 whole body will balance itself. 



88 When you balance a rod, a stick, or any other body, upon the finger, 

 where is the centre of gravity of the stick or body ? 



It is the point upon which the body will remain at 

 rest, or upon which it is balanced. 



89 In what position only can a body rest ? 



Only when its centre of gravity is supported ; and 

 until this is accomplished the body will move, and con- 

 tinue to do so, until it settles into a position in which 

 the centre of gravity cannot sink lower. 



0O Why does a person carrying a weight upon his back stoop forward ? 



In order to bring the centre of gravity of his body 

 and the load over his feet. 



Fig 1. 



fifr 2. 



