GRAVITY OF BCDIES. 



11 



therefore, as the square of 60 is 3600, a body will 

 weigh 3600 times less at that distance than upon 

 the surface of the earth : so that a body which 

 weighs a pound here would, at the distance of the 

 moon, be literally lighter than a feather. 



In geology we frequently have to speak of the 

 specific gravity of bodies, which is the weight of a 

 body compared with the weight of another body of 

 equal bulk ; which latter is regarded as the standard 

 of comparison. The standard generally employed 

 is rain or distilled water, a cubic foot of which 

 weighs 1000 avoirdupois ounces. A balloon as- 

 cends in the air, because the gas with which it is 

 inflated is specifically lighter than the atmosphere in 

 which it floats; and apiece of board* swims on the 

 surface of water, because its specific gravity is less 

 than that of water ; in other words, it occupies less 

 space than a body of water of the same bulk. For 

 the same reason, a piece of lead or iron will swim 

 in a vessel filled with quicksilver, while a piece of 

 gold will sink. In the one case, the specific gravity 

 is less, in the other, greater than that of quicksilver. 

 To find, then, the specific gravity of any body, we 

 have only to weigh it in air, which is its absolute 

 weight, and then in water, which is its relative 

 weight, and then compare the two results. Thus, 

 suppose a stone weighs in air 390 grains ; in water, 

 as it will be partly supported, it will weigh less by 

 80 grains than it did in air : and this difference is 

 precisely the weight of the water displaced by the 

 stone, or a quantity of water equal to the bulk of 

 the mineral. Now we get the specific gravity by 



* Mr. Scoresby has proved that the most porous, and, there- 

 fore, the lightest kinds of woods may, by being sunk deep below 

 the surface of the ocean, acquire a specific gravity so great as 

 to cause them to sink in water hke lead. He tried this experi- 

 ment with deal and other light kinds of wood. This phenom- 

 enon is owing to the immense pressure which the wood sustains 

 by the water. 



