MECHANICS 71 



the difference, the product will be what is called the 

 specific gravity of that solid. Let us suppose a solid 

 weighs 75 grains in water, and 80 in air; then 80 - 75 = 5 



and 1 6. The proportion, therefore, of the weights of 



equal bulks of the solid in question and of water, will be 

 80 to 5 or 1 6 to r, so that it will be 16 times heavier 

 than its own bulk of water, and the specific gravity of 

 that body will be 16. 



In England it is customary, for convenience, to con- 

 sider one cubic foot as the standard volume, and to ex- 

 press the weight in avoirdupois ounces or grains, and a 

 cubic foot of rain-water weighs about 1,000 oz. The 

 specific gravities of liquids may be ascertained by using 

 a vessel capable of holding 1000 grains of water at a 

 temperature of 60 Fahrenheit. 



If two liquids which differ in density such as water 

 and alcohol, or water and mercury be made to com- 

 municate in a vessel with two upward prolongations, or 

 limbs, then the height to which they will rise in the two 

 limbs will differ inversely as their densities the more 

 dense being the less high.* Thus two inches of mer- 

 cury will balance 27 inches of water. 



We have said t that the surface of a liquid is hori- 

 zontal, "apart from any disturbing influence"; but 

 certain attractions, other than the earth's gravity, 

 may interfere, more or less, with the horizontality of a 

 fluid's surface. 



Thus there is, of course, an attraction between the 

 vessel holding a liquid and the adjacent portion of such 

 liquid. If the solid vessel be denser than the liquid 



* As to " osmosis," another effect, of placing in proximity, 

 p. 146. t See ' P- 68. 



