GRAVITY OP BODIES. 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 hterally 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 speciflc 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 difi'erence 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 like lead. He tried this experi- 
ment with deal and other hght kinds of wood. This phenom- 
enon is owing to the immense pressure which the wood sustains 
by the water. 
