GRAVITY 



2580 



GRAY 



inspiration. He would take the crown and put 

 it into a vessel filled with water and note 

 how much spilled over. Then he would take 

 the same weight of pure gold and put it into 

 a vessel containing exactly the same amount of 

 water. If the crown were also of pure gold 

 it would be exactly equal in bulk to the piece 

 of gold and the amount of water displaced 

 would be exactly the same in both cases. But 

 if the gold in the crown were mixed with silver, 

 it would be greater in bulk than the piece of 

 pure gold, and the amount of water spilled 

 over would show it. In short, Archimedes had 

 discovered how to determine the specific grav- 

 ity of a substance. 



Specific gravity is the heaviness of a given 

 quantity of a substance as compared with that 

 of an equal quantity of another substance used 

 as a standard. Water is usually taken as the 

 standard of comparison for solids and liquids, 

 while air is taken as the standard for gases. 

 The specific gravity of a solid or liquid is the 

 ratio between its weight in air and the weight 

 of an equal volume of water; this ratio is se- 

 cured by dividing its weight in air by the 

 weight of an equal volume of water. 



Archimedes' principle, that a body immersed 

 in a fluid displaces an amount of the fluid 

 equal to its own loss of weight, furnishes the 

 most convenient method of finding the specific 

 gravity of a substance. First weigh the sub- 

 , stance in air; then weigh it in water; then 

 subtract the weight in water from the weight 

 in air. The result will be the weight of the 

 volume of water displaced. Divide the weight 

 of the substance in air by its loss of weight in 

 water and you will have the specific gravity of 

 that substance. 



A piece of brass is found to weigh seventeen 

 pounds in air and fifteen pounds when immersed 

 in water. Fifteen from seventeen leaves two (the 

 weight of an equal volume of water). Seventeen 

 divided by two equals 8.5 ; therefore, 8.5 is the 

 specific gravity of brass. 



To find the specific gravity of a body lighter 

 than water, weigh the body in air. Then take 

 a sinker heavy enough to keep the body under 

 water, and weigh the sinker in the air and in 

 the water. Then weigh both the body and 

 the sinker in water. By subtracting the loss 

 of weight of the sinker in water from the loss of 

 weight of the two bodies together, the loss 

 of weight of the light body is obtained. Divide 

 the weight ' of the light body by its loss of 

 weight in water and the result is its specific 

 gravity, which will, of course, be less than 1. 



A block of wood weighs 12 ounces in the air, 

 and a sinker 30 ounces. Together they weigh 42 

 ounces. The sinker weighs 25 ounces in water, 

 the two together weigh 7 ounces in water. Sub- 

 tract 7 (the weight of the two in water) from 42 

 (the weight of the two in air), and the remainder 

 is 35 (ounces), the loss of weight of the two ob- 

 jects in water. From 35 subtract 5" (the loss of 

 weight of the sinker in water), and the remainder 

 is 30 ounces, the loss of weight of the body in 

 water. Divide 12 (weight of body in air) by 30, 

 and the quotient is 0.4, the specific gravity of the 

 piece of wood. 



In finding the specific gravity of a liquid, a 

 specific gravity bottle is commonly used. This 

 bottle is made to hold a certain weight of 

 water, say a thousand grains. Fill the bottle 

 with the liquid to be tested and weigh it. Di- 

 vide the weight of the liquid in the bottle 

 by the weight of water it is made to contain 

 (1,000 grains), and the result is the specific 

 gravity of the liquid. The specific gravity of 

 a liquid may also be found by the use of a 

 hydrometer (which see). C.R.M. 



GRAY, ASA (1810-1888), recognized as the 

 foremost American botanist of his day, was 

 born at Paris Furnace, N. Y. After a few 

 years spent in the practice of medicine he 

 became curator of the New York Lyceum of 

 Natural History. In 1838 he was appointed pro- 

 fessor of natural history in the University of 

 Michigan, and in 1842 accepted a like appoint- 

 ment at Harvard College. As the leading 

 disciple of Darwin in the United States, he 

 advocated and defended the theory of evolu- 

 tion as being in harmony with the strictest re- 

 ligious views. The collection of his brilliant 

 discussions on the subject is entitled Darwinia. 

 Professor Gray's writings include many valu- 

 able textbooks on American flora, some of 

 which are Field, Forest and Garden Botany; 

 How Plants Behave; New Flora oj North 

 America, etc. No other botanical textbooks 

 than Gray's were used for years in the high 

 schools of America. See DARWIN, CHARLES. 



GRAY, ELISHA (1835-1901), an American in- 

 ventor for whom his friends claimed the honor 

 of invention of the telephone, denying the 

 credit to Alexander Graham Bell. He was 

 born at Barnesville, Ohio, and attended Oberlin 

 College, maintaining himself there by working 

 as a carpenter. Beginning his investigations in 

 1867, he took out nearly fifty patents for tele- 

 graph, telephone and other electrical appli- 

 ances, including a type-printing and a writing 

 or copying telegraph. In 1876 he filed in the 

 Patent Office specifications for a telephone, 

 but the patent was finally awarded to Alex- 



