SPECIFIC GRAVITY. The weight of any body, or substance, compared with the weight of some other body which is assumed to be a standard. The standard of comparison, by common consent and practice, is rain water, on account of its being less subject to variation, in different circumstances of time and place, than any other body, whether fluid or solid. A cubic foot of rain water weighs 1000 ounces avoirdupois ; therefore, assuming this to be the spe cific gravity of rain-water, and comparing all other bodies with it, the same numbers which express the specific gravities of bodies, denote, at the same time, their weight per cubic foot, in avoirdupois ounces. Hence, by reference to the tables, we are enabled to find the magnitude of any solid which is too irregular to admit of the common rules of mensuration, and also the weight of any body of known magnitude, which is too ponderous to be submitted to the operation of the steel-yard or balance.
.Example.—Required the quantity of material in an irregular shaped block of marble, weighing 4f tons.
Reduce the weight to ounces, and divide by the specific gravity of marble. Hence 4i tons X 16 oz. +2.838 = 56.8 cubic feet.
Required the quantity of material in a statue of white Parian marble, weigh ing 800 pounds.
800 X16=12800+ 2.838 = 44 cubic feet.
Again.—Required the weight of a block of Aberdeen granite, measuring 43 feet in length, 8 feet in breadth and thickness.
x 8 x 8=2752.
Then, as I :: 2752 :: 2.625 : 7229000=201 tons 11 cwt. 1 qr.
The properties of specific gravity are as follow : 1. A body immersed in a fluid will sink if its specific gravity be greater than that of the fluid ; if it be less, the body will rise to the top, and be only partly immersed ; and if the specific gravity of the solid and fluid be equal, rt will remain at rest in any part of the fluid in which it may be placed.
2. When a body is heavier than a fluid, it loses as much of its weight, when immersed, as is equal to a quantity of fluid of the same magnitude.
3. If the specific gravity of the fluid be greater than that of the body, then the quantity of fluid displaced by the part immersed, is equal to the weight of the whole body. Hence, as the specific gravity of the fluid is to that of the body, so is the whole magnitude of the body to the part immersed.
In concluding, it may be necessary to remark, that all bodies expand by heat and contract by cooling; but the contraction and expansion, by the same change of temperature, is very different in different bodies. Water, when heated from 60 to 100 degrees, increases its volume nearly one sixty-seventh of its bulk ; mercury, one two-hundred and forty-third part; and many substances much less. It is therefore proper, in ascertaining the specific gravities of bodies, to note particularly the temperature.