32 WELLS'S NATURAL PHILOSOPHY. 



free to move, furnish us evidence of this attraction, in tlie phenomena of the 

 tides. "When, by the revolution of the earth, a certain portion of its surtace 

 is brought witliin the direct influence of the moon's attraction, the siirface of 

 the ocean is attracted, or drawn up, to form a wave. This wave, or elevation 

 of the surface of the water, occurring uniformly, is called a tide ; when tha 

 moon is the nearest to the earth, its attraction is the greatest, and at these 

 periods we have high tides, or " high water." 



..^ , . „ 59. All bodies upon the earth are attracted 



What IS Ter- , ^ , 



restriai Gray- toward its Center. This we call Terrestrial 

 ' Uravitation. 



What is the The attraction of the earth is not the same 

 etrth's^attrac- at all distanccs from the center, being greatest 

 **°"^ at the surface, and decreasing upward as the 



square of the distance from the center increases, and down- 

 ward simply as the distance from the center decreases. 



SECTION I. 



"WEIGHT. 



„ . ^ , 60. When a body falls to the earth, it de- 



How is a body , . •' ' 



at rest upon sccuds becausc it IS attracted toward the center 



tbesurfaeoof . r- /> 



the earth at- of the earth. When it reaches the suiiace oi 



tracted? i i i • > i 



the earth, and rests upon it, its tendency to 

 continue to descend toward the center is not destroyed, 

 and it presses downwards with a force proportioned to the 

 degree by which it is attracted in this direction. Thia 

 pressure we call Weight. 



What is 61. Weight is, therefore, the measure of 



Weight? £^^^g ^-^j^ which a body is attracted by the 



earth. In ordinary language, it is the quantity of matter 

 contained in a body, as ascertained by the balance. 



"Weight being, then, the measure of the earth's attraction, it 



How does follows that as the attraction of the earth varies, weight must 



Weight vary? . , 



also vary, or a body will not have the same weight at all 

 places. 



The weight of a body will be greatest at the surface of the 

 Where ■^■'1 a earth, and greatest at those points upon the surface which are 

 the most, and nearest the center. 

 where the ^g ^^^ ^^^-^^ jg ^^^ ^ perfect sphere, but flattened at the 



poles, the poles are nearer the center than the equator. A 



