SPECIFIC GRAVITY, or more properly Specific. Weight, is the weight of any gas, liquid, solid, under some given volume, as a cubic foot, a cubic inch, die. Distilled water is the substance usually employed for the purpose of comparing together the weights of all substances except the gases ; and because the volume of any substance varies with its temperature, in determining from experiment the specific gravity of any substance, the weight under a given volume is reduced to that which it would become at one constant temperature.
In the Parliamentary Regulations, which were made in 1825, a cubic inch of water is stated to weigh 252.458 troy grains, the tem perature being 62° Fahrenheit, and the height of the barometrical column 30 inches ; and 7000 troy grains are made equivalent to one pound avoirdupois ; hence it follows that a cubic foot of water should weigh 997.136 ounces. This number is sufficiently near 1000 to make it very proper that it should be adopted for the specific gravity of water, since a change in the value of the avoirdupois ounce, which would be scarcely appreciable in the ordinary trans. actions of commerce, would render the ounce an accurate and convenient unit of weight, while the cubic foot constitutes the unit of volume.
On the Continent, since the employment of the decimal scale of weights and measures has become general, the cubic centimetre (.061028 cubic-inches English) is the unit of volume, and the gramme (15.407 troy grains) is the unit of weight; the gramme having been de termined by the weight of a cubic centimetre of distilled water of the temperature at which its densityis a maximum• (38.25° Fahrenheit).
The hydrostatic balance which is employed in determining the specific gravities of bodies is constructed similarly to an ordinary balance, but with as much delicacy as possible; and a particular contrivance is adopted in order to ascertain the weight of any substance within, for example, one-hundredth part of a grain. Thus, from the lower part of the scale co/1z taining the substance to be weighed, there is suspended a brass wire, whose volume and weight have been previously determined, and part of its length enters into water which is contained in a vessel underneath the scale: The scales with this wire thus attached to one of them being previously put in equilibrio when the surface of the water is at a certain mark on the wire, the substance to be weighed is introduced into the scale above the wire, and weights are placed in the opposite scale till one grain more would be found too great : then gently raising the whole balance till, by the increase of the weight on the side of the scale containing the substance, in consequence of a greater portion of the. wire being out of
the water, an equilibrium takes place. The wire being graduated so that 100 divisions cor respond to a weight equal to one grain, the number of graduations on it between the sur face of the water and the fixed. mark before mentioned will enable the . experimenter to determine the number of hundredths of a grain by which the weight of the substance in the scale exceeds the number of grains already placed in the opposite scale.
The weighing process varies in its details according to the kind of substance to be weighed. Thus—a solid which is heavier than water, a solid which is lighter than water, a solid which is soluble in water, a solid which will imbibe water without being dissolved, a solid in the form of sand or powder, a liquid, a gas or vapour—all require peculiar methods for determining their specific gravity; methods which are fully described in treatises on Hydro statics, but which we need not dwell upon here.
The following are the specific gravities of several airs and gases, at a temperature of a lateral direction, without giving unnecessary weight.
The Periscopic Spectacles of Dr. Wollaston were contrived in order to allow considerable latitude of motion to the eyes without fatigue, by conforming the shape of the glasses to that of the eyes. This is effected by the use of lenses either of a meniscus or a concavo-con vex form, the concave side being in both cases turned towards the eye. Divided spectacles, each glass consisting of two half-lenses, are sometimes used, the upper half of each glass being occupied by a concave lens, or one of very slight convexity, for seeing distant ob jects, while the lower half has a strong rang, nifier, for examining things near the eye. The present Astronomer Royal finds that one of his eyes refracts the rays of light to a nearer focus in the vertical than in the horizontal plane; and this defect he has succeeded in' remedying by using a double concave lens, one surface of which is spherical and the other cylindrical.
As an article of manufacture, the best spec tacles are made by the leading opticians of London ; but the medium and inferior kind are made on a large scale in Birmingham.