26 Specific Gravity. [Book I. 



more of folid matter than the feather, and confequent- 

 ly requires a more confiderable force to put it in mo- 

 tion, yet it appears that the attractive power being 

 proportioned to the quantky of matter, its velocity is 

 equal to that of a body which requires lefs force to put 

 k in motion. 



Secondly, The attractive force of bodies is recipro- 

 cally as the fquares of the difcances. Thus, if a body 

 is of the weight of one hundred pounds at the diftancc 

 of ten diameters of the earth, at half that diftance it 

 would have four times that weight, or the force of gra- 

 vity would be exerted upon it in a quadruple ratio, 

 and fo in proportion as it approaches the body of the 

 earth. 



It would perhaps have been more correct to have 

 fpoken of what is commonly called fpecific gravity in 

 treating of the denfities or porofity of bodies, but the 

 rea'fon why it was omitted on that occafion will pre- 

 fently be apparent. The truth is, we have no mode 

 of determining the dcnfiry of bodies, but by the firfl 

 of thefe laws of gravitation, which have juft been no- 

 ticed. For fmce the force of attraction which nature 

 exerts upon all bodies, is in proportion to the quantity 

 of matter which they contain, it follows of courfe, that 

 if, of two bodies equal in bulk, the one is heavier than 

 the other, that body is poflHTed of greater denfity, or 

 contains more matter in the fame compafs. 



Thcfpecifa gravity is therefore the very fame thing 

 with the denfity of bodies, and has relation to the quan- 

 tity of folid matter which different bodies contain in 

 the fame bulk. J t is alfo called relative or compara- 

 tive gravity, becaufe we judge of it by comparing one 

 body with another. If bodies are equal in bulk, it is 

 evident their fpeciiic gravities may be eafily deter- 

 mined by a common balance, and hence fluids, or any 



fubftances 



