CENTRE OF GRAVITY. J^ 



nerally agreed that the same principle of gravity, by whicll 

 we see all bodies tend toward the centre of the earth, is m 

 general law of nature, extended to all distances, and to every 

 body, or substance, in the universe. 



For this the moon thro' heaven's blue concave glides, 



And into motion charms th' expanding tides, 



While earth impetuous round her axle rolls. 



Exalts her wat'ry zone, and sinks the poles. — Falconer. 



Questions. — 1. What is the attraction of gravitation ? 2. How 

 i« the tendency of bodies to fall produced ? 3. Wliat incident led 

 Newton to the most wonderful discoveries ? 4. How did he reason ? 

 5. What is considered the mark of a superior genius ? 6. What did 

 Newton soon perceive respecting the force of gravity ? 7. What did 

 this lead him to conjecture ? 8. How did he reason respecting the 

 moon? 9. What had this principle of gravitation been styled? 10. 

 What did Newton fullj{^xplain by it ? 



LESSON 17. 



Centre of Gravity. 



Perpendic'ularly , in the direction of a straight line up and down. 

 Pyr'amid, a pillar ending in a point. 



The centre of gravity of a body is that point about which 

 all its parts, in any situation exactly balance each other, so 

 that if a body be suspended or supported by this point, it will 

 rest in any position. Whatever supports the centre of gra- 

 vity bears the weight of the whole body ; and while it is sup- 

 ported the body cannot fall. We may consider, therefore, 

 the whole weight of a body as centered in this point. If a 

 line is drawn from the centre of gravity of a body, perpen- 

 dicularly to the horizon, it is called the line of direction; 

 because it is the line whicti the centre of gravity would de- 

 scribe, if the body fell freely. The broader the base is 

 upon which a body rests, the more difficult it will be to over- 

 turn it, as it must bo moved the more to bring the line of 

 direction beyond the base. A cask is easily rolled along, 

 and so is a ball, but a box is moved with greater difficulty. 

 When a box is longer than it is broad, it is much more easily 

 turned on its side than set on its end. A building in tho 



