84 WELLS'S NATURAL PHILOSOPHY. 



What would be hours, the Centrifugal Force -would be equal to the attraction 



Telocity of ro- o*" gravitation, which may bo considered as the Centripetal 



tation of the Force, and aU bodies at the earth's equator would be deprived 

 earth were »n- ^ . , . . ^, ,, , , , 



creased ? 01 weight, since they would have as great a tendency to leave 



the surface of the earth as to descend toward its center. If 

 the earth revolved on its axis in less time than 84 minutes, terrestrial gravita- 

 tion would be completely overpowered, and all fluids and loose substances 

 •would fly from its surface. 



173. There appears to be a constant tendency to rotary 

 motion in moving bodies free to turn upon their axes. 

 The earth turns upon its axis, as it moves in its orbit ; a 

 ball projected from a cannon, a rounded stone thrown from 

 the hand, all revolve around their axes as they move. 



TlQ. 58. This phenomenon may be very 



prettily illustrated by placing a 

 watch-glass upon a smooth plato 

 of glass, Fig. 58, moistened suf- 

 ficiently to insure slight adhesion, 

 and fixed at any angle. As it 

 ^*— begins to move toward the bot- 

 tom of the inclined plane, it will exhibit a revolving motion, which uniformly 

 increases with the acceleration of its downward movement. 



PRACTICAL QUESTIONS AND PROBLEMS ON THE PRINCIPLES 

 AND COMPOSITION OF MOTION. 



1. The STmrACK of the eabth at the equatob mores at the rate of about a TnoTJBAirr 

 MILES in an houk : why are men not sensible of this rapid movement of the earth ? 



Because all objects about the observer are moving in common with him. It 

 13 the natural uniformity of the undisturbed motion which causes the earth 

 and all the bodies moving together with it upon its surface to appear at 

 rest. 



2. How can you easily see that the eabth is in motion? 



By looking at some object that is entirely unconnected with it, as the sun 

 or the stars. "We are here, however, liable to the mistake that the sun or 

 atara are in motion, and not we ourselves with the earth. 



3. Does the bun really eise and bet each day ? 



The sun maintains very nearly a constant position ; but the earth revolves, 

 and is constantly changing its position. Really, therefore, the sun neitJier rises 

 nor sets. 



4. Why, to a pebson sailing in a boat on a smooth stream, or ooino bwiftlt- in a 

 OABBIAG on a smooth road, do the trees or buildings on the banks or roadside appear to 

 move in an opposite dieection ? 



The relative sUiiation of the trees and buildings to the person, and to each 



