50 



GRAVITATION. 



The force of gravity tends to draw every particle of 

 matter toward the centre of the earth, or downward in a 



vertical line. We may therefore 

 consider the effect of this force 

 upon any body as the sum of an 

 almost infinite number of paral- 

 lel forces, each of which is acting 

 upon one of the molecules of 

 which that body is composed. 

 We may also consider this sum 

 of forces, or total gravity, as 

 acting upon a single point, just 

 FIG. 18. as the force exerted by two 



horses harnessed to a whiffle- 



tree is equivalent to another force (resultant) equal to the 

 sum of the forces exerted by the horses, and applied at a 

 single point at or near the middle of the whiffle-tree. 

 This single point, which may thus be regarded as the 



point of application of the 

 force of gravity acting upon a 

 body, is called the centre of 

 gravity of that body. In other 

 words, the weight of a body 

 may be considered as concen- 

 trated at the centre of gravity. 



1O8. How to find the 

 Centre of Gravity. In 



a freely moving body, the cen- 

 tre of gravity will be brought 

 as low as possible, and will, 

 therefore, lie in a vertical line 

 FIG. 19. drawn through the point of 



