GRAY
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GRAY
the moon) that retains the moon in her orbit. He proceeded to compute the period of revolution of the moon on this hypothesis, and found, after getting correct data, that this period is very exactly that of the lunar month.
Later he showed that the attraction of the sun on the planets, according to the same laws, would account for all the planetary motions. Recent observations apparently show that the ordinary laws of gravitation hold even in the case of double stars, bodies entirely outside the solar system. The principal facts known concerning gravitation are the following: i. The force of attraction between any two particles varies directly as the product of the masses of those particles. 2. The force of attraction between particles varies inversely as the square of the distance separating these particles. 3. The force of attraction is independent of the kind of matter composing the particles and independent of the medium between the particles.
These laws may all be summarized in the following equation. Let m and ml represent the masses, r the distance separating them and F the force of attraction. Then
where G is a constant known as the gravitation constant. The exceedingly accurate measures of C. V. Boys give for this constant the value, 6.6$j6-x.io*8. The equation for F, just given, is known as Newton's Law of Universal Gravitation. No satisfactory explanation of gravitation has ever been suggested. Newton pointed out very clearly that his law describes the facts, but does not explain them. Among the most important inferences which may be drawn from Newton's law are: (i) Kepler's three laws; (2) the fact that a homogenous sphere acts as if its entire mass were concentrated at its center; and (3) the attraction of a homogenous sphere on a particle lying anywhere between its center and its surface varies directly as the distance of the particle from the center.
Gray, Asa, American botanist, was born at Paris, N. Y., Nov. 18, 1810; became a physician in 1831; but soon abandoned medicine for the study of botany. He was appointed botanist of the United States exploring expedition to the southern seas in 1834, but resigned before setting out in 1837. In 1842 he became professor of natural history at Harvard, and remained until 1873. In 1874 he became regent of Smithsonian Institute, succeeding Agassiz. Besides many textbooks, now in use in all schools, he wrote Examination of Darwin's Treatise; Darwinia', and Natural Science and Religion. He died Jan. 30, 1888. He ranks among the leading botanists of the age. He was very successful as a teacher and through his books in impart-
ing the first principles of botany to beginners. He also was the first in America to arrange the species of plants in accordance with the natural system of arrangement which has taken the place of the older system of Linnaeus.
Gray, Horace, American jurist and associate-justice of the supreme court of the United States, was born at Boston, Mass., in 1828, and graduated from Harvard. After studying at Harvard Law School he was admitted in 1851 to the Massachusetts bar, and for seven years (1854-61) he acted as reporter to the supreme court of the state. From 1873 t° I&82 ne was chief-justice of the supreme court of Massachusetts, and in the latter year was elevated to the supreme court of the United States, as one of the associate-justices. He died on Sept. 15, 1902.
Gray, Thomas, one of the foremost English poets, was born at London, Dec. 26, 1716. His father's violent temper had forced his mother to leave him, and Thomas was brought up by his mother and her sister, of both of whom he was passionately fond. At school, at Eton, he met his best friend, Horace Walpole, who at once took a fancy to the shy and studious boy. At Peter-house, Cambridge, he greatly disliked mathematics, which then was the chief study there, and after some four years of university work he was easily persuaded to spend two years and a half in travel with Walpole. On reaching home he began to write poetry. The next spring (1742) he went back to Cambridge, took his degree in civil law, and as a resident-student spent there the most of his life, though he afterward left Peterhouse for Pembroke Hall, because of a practical joke played on him by some of the students. He took the keenest pleasure in the company of his friends. His holidays were spent with his mother at Stoke Pogis or with Walpole at London. His Ode on Eton College was printed in 1747. The death of his aunt seems to have brought his mind back to a projected work which was soon to bring him fame — his Elegy in a Country Churchyard. This poem he had begun on his return from the trip with Walpole, and had then thrown aside. The poem was sent to Walpole, was handed about in manuscript, and soon became so well-known that Gray was forced to print it in 1751. This stately poem is perhaps the best-known piece of English verse. Wolfe said he would rather have written it than capture Quebec. Three years later appeared his Pindaric Odes and Progress of Poesy. Perhaps his greatest work followed after another three years had gone by — The Bard. His studies of Icelandic and Celtic verse bore fruit in The Fatal Sisters and The Descent of^Odin. In 1768 he was made professor of history and modern languages at Cambridge. Dr. John-
