NATURAL PHILOSOPHY. 



[Lesson XI. 



quently the central point of parallel and 

 equal forces. 



141. T. If a body is not suspended with 

 clue regard to the centre of gravity, what 

 will be the result? 



P. It will move until it settles in a po- 

 sition in which the centre of gravity cannot 

 fall lower. 



142. T, What is the use of finding out 

 the centre of gravity ? 



P. When we know the weight of bodies 

 and their centre of gravity, we can substi- 

 tute the weight for all the forces acting 

 upon the body, and a single point (the 

 centre of gravity), for the collective points 

 forming the body. 



143. T. Have all solid bodies a centre 

 of gravity ? 



P. Yes ; the centre of gravity is sure 

 to be in the centre of all round, square, and 

 regular bodies of the same density ; and 

 no mattr what form a body has, it is sure 

 to have a centre of gravity. In irregular 

 bodies it is situated at that point which 

 will place the body in a state of equilibrium 

 when fixed or suspended from it. 



144. T. Can I stand an egg, or place 

 it in a state of equilibrium, upon its narrow 

 end? 



P. Not without following the example 

 of Columbus. Because, if there is not any- 

 thing to support 

 the egg, it will as- 

 sume such a posi- 

 tion, that a line 

 drawn from the 

 centre of gravity 

 to the point below, 

 where the body 

 Fig. 12. comes in contact 



with the surface, will be the shortest that 

 can be drawn from the centre to any other 

 part of its superficies. If you observe this 

 diagram carefully, you will see that the egg 

 could not remain in the position it is in 

 the figure; it would roll over, and instead 

 of the line a c being perpendicular to the 

 line d t, it would be the line c b. 



145. T. If the centre of gravity of a 

 body be a fixed point, the body itself will 

 always be in a state of equilibrium, no 

 matter how we turn or place it. Is this 

 so? 



P._Yes; the only thing that is actually 



necessary to the equilibrium of any body 

 is, that the centre of gravity should be 

 suppn: 



146. T. How will you find the centre of 

 gravity of an irregular 

 tigure ? 



P. Very easily ; or 

 that of any body. If 

 you suspend the body at 

 a point a, (Fig. 13,) the 

 direction of the string 

 supporting it will pass 

 through the lower part 

 of the body at l>, and 

 therefore it is very evi- 

 dent the centre of gra- 

 vity is in the direction 

 of the line a b. Again, 

 suppose we suspend the 

 same body at the point 

 d (Fig. 14), the centre 

 of gravity is still in the 

 direction of the sus- 

 pending string de, and therefore the ci litre 

 of gravity lies where the two lines a b and 

 d e cross one another at c. Although it 

 is easy to determine the centre of gravity 

 of some bodies of regular form and uniform 

 density by geometric principles, yet there 



Fig. 13. 



Fig, 14. 



are certain bodies in which it is difficult to 

 ascertain the line of direction exactly. 



147. T. What do you mean by the line 

 of direction? 



P. It is the imaginary line drawn from 

 the centre of gravity of a body toward the 

 centre of the earth ; therefore it is evident 

 that, if the line of direction fall within the 

 base of any body, it will stand ; if not, the 

 body will fall over, or, what is generally 

 termed, over-balance itself. 



