NATURAL PHILOSOPHY. 





falling of an apple; and the direction of 

 rivers to find their lowest level, are all the 

 effects of the force of gravitation. 



80. T. How can you ascertain the di- 

 rection of the force of gravity ? 



P. Fasten a string at one extremity (). 

 and attach a small heavy 



r^ Jit (w) to the other end 



of the string. The direc- 

 tion of the string, when it is 

 :oned and at rest, will 

 give the direction of gravity 

 v with accuracy. The weight 

 (a) is called a plummet ; and 

 the line which the string (t>) 

 forms in a state of equili- 

 brium, the vertical line. 



81. T. Can you always 

 determine the direction of 

 9 ' 7 ' gravitation ? 



P. Yes ; by means of the plummet, it 

 may easily be ascertained at all times and 

 in all places. 



82. T. Where is the force of gravity 

 always directed ? 



P. Towards the centre of the earth. 



83. T. If this is the case, the direction 

 of the plummet must be different in various 

 parts of the earth. 



P. So it is ; the directions of the plum- 

 met. at two different parts of the earth, are not 

 ike a certain angle 

 lie point of which coin- 

 cides with the central point of the earth. 



r. Can you explain to me how it 

 is that bodies do not descend in parallel 

 .is tbcy appear to do ? 

 1 have a diagram here that will ex- 

 plain it very easily. Let A u c i , 

 a9 CJ 5 apart ofil 



,cc of 

 'ih, and 

 a & 6 two balls 

 of lead. 



these balls are 

 left to 



1 in the 

 \ and b 



was POK.V 

 the balls to descend to the < 

 earth they would meet at o, the same as 



** 



the lines do. If the ball c was let fill at 

 that part of the globe, it would descend in 

 a straight line (c c) so as to form a right 

 ith the line a A. 



/'. Is gravity a general property of 

 allbodies ? 



PS Yes ; fluids, solids, and gaseous 

 bodies are alike subject to its laws. 



86. T. Suppose that the body is inter- 

 rupted in its descent by some other sup- 

 porting body, what is the result ? 



P. The action of the force of gravity 

 does not cease ; but expends itself by pres- 

 sure upon the object intervening. 



87. T. What is the weight of a body ? 



P. It is the amount of pressure exer- 

 cised by one body upon another body on 

 which it rests ; the pressure increasing with 

 the number of the material particles of 

 the body. 



88. T. What is the mass of a body' 

 P. It is the quantity of matter of which 



it is composed ; and therefore the larger 

 the quantity of matter, the greater the 

 resistance it will offer. 



89. T. You have stated that gravity is 

 the force which compels bodies to fall. K\- 

 plain to me how it is that smoke, clouds of 

 vapour, and air-balloons ascend, if the theory 

 of the attraction of gravitation is cor 



P. These are only additional proofs of 

 the effect of gravit i 



.tponr, and balloon are 

 upwards solely by the force of the 

 spheric air through which they pass; as 

 they are lighter than the air, they ascend, 

 ami displace or thrust down portions of air 

 10 their volume. 



90. T. This does not prove to me that 

 they are in!' 



gravitation: expl ,iore fully f 



P. All substan 



the atmospln irface of the 



earth, v, ich that part 



of the air that is not so dense ; and as two 



bodies cannot occupy the same space at 



the sail! ke, vapour, or bal- 



! air equal to 



ilk. The s:no',. 



'I till its particles unite 

 and form flakes of soot, which 

 air, dcicrnd; n 



