4? 6 Recapitulation of Principles, fcfr. [Book VII. 



the cavity of the body is made fo, that the bulk may 

 be a little larger than the magnitude already found, the 

 body will have lefs weight under the lame magnitude 

 than the fluid. For example, fuppofe that it is re- 

 quired to make a ball of iron of thirty pounds, and of 

 fuch a magnitude as that it will fwim upon water. 

 Sir.ce the weight of a cubic foot of water is feventy 

 pounds, a volume of water weighing thirty pounds 

 \vill be r!;ree- fevenths of a cubic foot: hence it will be 

 eafy to find what the diameter of a fphere muft be, the 

 fetid contents of which mail be three- fevenths of a 

 cubic foct. Therefore, the ball of iron muft be made 

 hollow within, and in fuch a manner that its diameter 

 may be greater than the diameter of a fphere, the folid 

 contents of which is three-fevenths of a cubic foot: if 

 this ball is made eleven inches three lines in diameter, 

 it will fwim. Hence we fee that it is not neceiTary for 

 a body to fwim, that it mould be compoled of matter 

 fpecin'cally lighter than water j it fuffices that its bulk 

 .or volume mould be greaf and its gravity fmall : 

 for though copper is about eight times as heavy as 

 water, yet military men have occafionally employed 

 boats of copper in conftructing bridges for the paffage 

 of troops. 



A copious table of fpecific gravities is given in the 

 Appendix to this volume. 



