WEIGHT. There is nothing to say on the feeling of weight after what has been said in PRESSURE; nor is it possible to give any idea which will be half so good as that which presents itself In raising a heavy body from the ground. The measure of weight is weight itself [BALANcs], and two weights are equal which counterpoise each other when placed at the ends of equal arms of a self-poising lever.
The weight of a body, that is, of a given bulk of known substance, 1.8 referred to that of water by what is called the SPECIFIC GRAVITY Of the substance. It is said, for example, that the specific gravity of ivory is 1826, when that of water is 1000. This means that any bulk of ivory is more weighty than the same bulk of water in the proportion of 1826 to 1000. When the specific gravity of water is called 1, that of ivory is 1-826. Since a thousand ounces avoirdupois of water aro nearly a cubic foot, a more popular notion of the meaning of specific gravity may be given, in this way :—To say that the specific gravity of a substance is 1126, that of water being 1, is to say that a cubic foot of it weighs 1.826 x 1000, or 1826 ounces nearly. More correctly, from 1000 times the specific gravity (water being 1), subtract three times that specific gravity, and add its 73rd part : the last step may be left out for common purposes. Thus, the specific gravity being 4117,
x 1000 — 4'817 x 3 is 4802'549, the number of ounces in a cubic foot.
But it is to be remembered, when weight is to be very accurately taken, that everybody is buoyed up to a certain extent by the air ; and the weight of a body in air is less than it would be in a vacuum by the weight of its own bulk of air. Now the air varies in weight [Ant] in a manner which may be ascertained nearly by the indications of the barometer. Properly speaking, it varies in a manner depeuding upon the superincumbent pressure, the temperature, and the quantity of moisture contained in it. A hundred cubic inches of dry air, when the barometer is at 30 inches and Fahrenheit's thermometer at 60°, weigh 31.012 grains. In measuring standards of weight, therefore, close attention must be paid to the state of the air at the time of weighing and to the substance weighed. If an iron weight balance a wooden one in a given state of the atmosphere, for that very reason there can not be strict equilibrium in any other state of the atmosphere; wood being at least seven times as bulky as iron, the effect produced on the weight of the wood by the alteration of the state of the air is at least seven times as much as that produced on the iron.