Page:The American Cyclopædia (1879) Volume VIII.djvu/185

 GRAVITY (SPECIFIC) 177 OBSERVER. ' Place. Latitude. Length of pendulum vibrating seconds, in inches. Velocity in feet ac- quired in one second by a body falling from rest. Sabine Spitzbergen N. 79 50' 39-21469 82-2528 Sabine 70 40 39 19475 82 "23 68 Stockholm '. ... 69 21 89-16541 82*2122 Bessel Konigsberg 64 42 39-15072 82-2002 Sabine 51 29 89-13983 82*1912 Borda Biot and Sabine Paris 48 60 89-12851 82*1819 Biot ... Bordeaux 44 50 89-11296 82*1691 New York 40 48 89-10120 82 "1594 Sandwich islands 20 52 89-04690 32'1148 gabine Trinidad 10 89 89 01888 82-0918 Kawak 8. 2 89 01433 82*0880 Ascension 7 55 89 02863 82 0956 Freycinet and Duperrey. . Mauritius 20 10 89-04684 82*1151 Brisbane and Rumker Paramatta 83 49 89-07452 82-1875 Frevcinet and Dunerrev. . . Falkland islands... 51 85 89-13781 82*1895 these values the following very simple and convenient formula has been deduced : If L denote the length of a seconds pendulum at any latitude A, and 39-017 inches be the igth of a seconds pendulum at the equator, YITY, Specific, the ratio of the weight of one body to that of an equal volume of an- other, adopted as a standard of reference. For solids and liquids the standard is pure water, at a temperature of 60 F., the barometer being at 30 inches. Air is the standard for aeriform bodies. A cubic foot of water weighing 1,000 oz., if the same bulk of another substance, as for instance cast iron, is found to weigh 7,200 pz., its proportional weight or specific gravity is 7'2. It is convenient to know the figures representing this proportion for every sub- stance in common use, that the weight of any given bulk may be readily determined ; and for all substances the specific gravity is used among other tests for the purpose of distin- guishing bodies from each other, the same substance being found, under the same cir- cumstances, to retain its peculiar proportional weight or density. Hence tables of specific gravity are prepared for reference, and in every scientific description of substances the specific gravity is mentioned. In practical use, the weight of a cubic foot is obtained from the figures representing the density by moving the decimal point three figures to the right, which obviously from the example above gives the ounces, and these divided by 16 the pounds avoirdupois, in the cubic foot. Different meth- ods may be employed to ascertain the specific gravity of solids. That by measuring the bulk and weighing is rarely practicable, nor is it desirable. As a body immersed in water must displace its own bulk of the fluid, the specific gravity may be ascertained by introducing a body, after weighing it, into a suitable vessel exactly filled with water, and then weighing the fluid which is expelled. The proportional weight is then at once obtained. Wax will cause its own weight of water to overflow; its specific gravity is then 1. Platinum, according to the condition it is in, will cause only from A- to FT? of its weight of water to escape, showing its specific gravity to be from 21 to 21*5. But a more exact method than this is commonly employed. The difference of weight of the same substance, weighed in air and when immersed in water, is exactly that of the water it displaces, and may consequently be taken as the weight of its own bulk of water. The spe- cific gravity then is obtained by weighing the body first in air, and then, suspended by a fibre of silk or a hair, in water, and dividing the weight in air by the difference. If the body is lighter than water, it is to be attached to one heavier, to make it sink; then find the loss of the two by immersion, and also the loss of the heavier body ; the difference will express the weight of water displaced by the lighter body, whose weight divided by this difference will give its specific gravity. It is hardly neces- sary to say that the substance examined must be free from mixture of foreign matters, and especially from cavities that may contain air. Minerals, if suspected to contain such, should be coarsely pulverized, and then the second method above may be conveniently applied to determine their density. The specific gravity of fine powders may be determined by one of the methods employed for ascertaining the spe- cific gravity of fluids, viz. : by comparing the weight of a measured quantity with that of the same quantity of water. A glass vessel called a specific gravity bottle is commonly employed, which is furnished with a slender neck, upon which is a mark indicating the height readied by 1,000 grains of water. The substance to be examined is introduced till it reaches the same mark, and, the weight of the empty bottle be- ing known, only one weighing is required to obtain the result. A common method for find- ing the specific gravity of fluids is by the in- strument called a hydrometer or areometer, of which several kinds are in use, all dependent on the principle that the weights required to im- merse a light body, as a bulb of glass, in different fluids, are proportional to the densities of these fluids. Such instruments are used for ascer- taining the specific gravity of liquors, as an in- dication of their strength. (See HYDROMETER.) Gaseous bodies are weighed in a thin glass flask or other vessel made for the purpose, and