Page:Popular Science Monthly Volume 3.djvu/113

Rh to be found is the one he there constructed in 1860, the circles being no less than six feet in diameter, and the telescope affixed between the two graduated disks being twelve feet long, and having an object-glass of as many as eight inches in aperture. Through this splendid apparatus the altitude of the stars, as well as the time of meridian passage, is now unerringly marked at the great national observatory. But the greatest of all the instruments established by him at Greenwich is a large, first-class equatorium, well known among astronomers.

During Sir George Airy's rule at the observatory he has, in the midst of his other labors, reduced the Greenwich observations of the moon and of the planets from 1750 down to the present tfme. Incidentally he has thrown considerable light on ancient chronology by his ingenious calculation of some of the most renowned of historical eclipses. Thrice the Astronomer Royal has taken occasion to visit the European Continent for the purpose of making more accurate observations upon the solar eclipse then eagerly anticipated. In 1854 he approximated more nearly than any previous investigation had done to the weight of the earth, through a series of experiments on the relative vibration of a pendulum at the top and bottom of Harton Coal-pit.

Sir George Airy has been repeatedly called into council on matters of grave difficulty by the government. He was chairman of the royal commission empowered to supervise the delicate process of contriving new standards of length and of weight, the old standards having been destroyed in 1834 in the conflagration of the Houses of Parliament. He was consulted some years afterward by the government in respect to the bewildering disturbance of the magnetic compass in iron-built ships-of-war. Thereupon he contrived an ingenious system of mechanical construction, through a combination of magnets and iron. The result was successful, and the system generally adopted. He conducted the astronomical observations necessary to the drawing of the boundary-line now traceable on the map of the New World between the Canadas and the United States. During the battle of the gauges in the railway world Sir George Airy strenuously advocated the narrow gauge, and he just as energetically advocates the adoption of a decimal currency. The writings of the Astronomer Royal are numerous. He has contributed largely to the Cambridge Transactions and the Philosophical Transactions. His pen has notably illustrated the memoirs of the Astronomical Society. He has written abundantly for the Philosophical Magazine, and still more abundantly, under his reversed initials, A. B. G., in the columns of the Athenœum. His principal works, however, are those which may be here rapidly enumerated: "Gravitation," published in 1837, was written originally for the "Penny Cyclopædia." "Mathematical Tracts" have reached a fourth edition, as have also his "Ipswich Lectures on Astronomy." In 1861 appeared his treatise on "Errors of Observation;" in 1869 his treatise on