Specific Gravities. , 309 



and V the bulk of the part immersed, the weight of this body 

 will be Sb ; and that of the fluid displaced will be S' b' ; whence 

 we have 



Sb = S'&', 

 an equation which gives 



A/ Sb 



&' = ^r; 



from which it will be seen, that the weight Sb of the body re- 

 maining the same, the part immersed will always be so much the 

 less, according as the specific gravity pf the fluid is greater. 



Moreover, this same equation is equivalent to the proportion, 



b : V :: S' : S- 



that is, the bulk of the body is to that of the part immersed, in- 

 versely as the specific gravity of the body to that of the fluid. 



428. If the immersed body weigh more than an equal bulk 

 of the fluid, it must sink ; and it can be retained only by a force 

 equal to the excess of its weight above that of an equal bulk of 

 the fluid. Now if we represent the specific gravity of the fluid, 



stance compared in point of weight with the same bulk of water 

 would evidently give the same result as any other measure ; and 

 correct results once obtained, we can substitute such other balks as 

 we choose. The cubic foot has this particular advantage, that, 

 if the measures be taken at the temperature of 50 of Fahrenheit, 

 the same numbers which express the specific gravities, the decimal 

 point being removed three places, or the whole being multiplied by 

 1000, give the absolute weight in avoirdupois ounces ; since a cubic 

 foot of water at this temperature weighs 1000 ounces. It is usual, 

 however, in determining specific gravities, to refer to the tempera- 

 ture of 60, at which a cubic foot of water weighs 62,353 lb * or a little 

 less than 1000 ounces. In the more accurate experiments, however, 

 upon this subject, the absolute weight is first ascertained, and the cubic 

 inch is taken as the measure, this bulk of distilled water at the tem- 

 perature of 60, being estimated at 252,525 grains. In France the 

 temperature preferred is that of the maximum density of water, or 

 39, 39. The atmosphere is employed as the unit in estimating the 

 specific gravities in gases. 



