HISTOBY OF PHYSICAL ASTRONOMY.

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astronomy by the great solar eclipse of 1748. He contracted an earlyintimacy witli Bradley, whom lie assisted in the computations connectedwith his researches on refraction. At the recommendation of that astro-nomer, he proceeded to the island of St. Helena , to observe the transitof Venus, in 1701. He commenced his labours at the Observatory ofGreenwich on the 7th of May, 1765. His instruments were the same asthose used by Bradley and Bliss, but his plan of observation was morelimited than that pursued by his predecessors. He confined his attentionto the sun, moon, and planets, and a select number of stars. The errorsof collimation of his quadrants were determined from time to time byobservations of y Draconis, with the zenith sector having its face turnedalternately east and west. During the intermediate intervals, the errorswere deduced by means of the quadrants themselves, from observations ofthe stars in the feet of the constellation Gemini. We have seen thatFlamsteed employed observations of the same stars, for the purpose ofdetermining the error of collimation of his mural arc. In order that hemight be enabled to deduce from his observations the absolute rightascensions of the sun, moon, and planets, it was necessary to establishwith great precision a definite number of reference points along the con-tour of the zodiac. With this view he selected thirty-six stars situatenear the equator, each of which was sufficiently bright to be observed inthe daytime, even when not very far removed from the sun. In order torender the observations of these stars available for ulterior purposes, itwas necessary to ascertain their places with respect to the equinox. Thiswas effected by comparing one of them, a. Aquila, with each of the re-maining thirty-five, and then comparing « Aquil® directly with the sun.The mode of investigation pursued by Maskelyne on this occasion wasfounded mainly upon Flamsteed’s method for determining the absoluteright ascension of a star. He continued to observe these stars on everypracticable occasion, and employed the results in the formation of a smallcatalogue which, in point of accuracy, far exceeded anything of the kindhitherto executed. By comparing the observed transits of these standardstars with the corresponding results deduced from the catalogue, it waspossible to ascertain the exact error of the clock, and hence to determinethe right ascensions of the sun, moon, and planets.

Maskelyne first introduced the practice of observing a star at the fivevertical wires of the telescope. This mode of observation was pursued byhim with undeviating regularity throughout his long career. He was alsothe first who noted the instant of transit in terms of the tenths of asecondHe continued to prosecute his labours without interruption,

.—adhering constantly to the same plan of observation which he hadtraced out in the beginning of his career—till his death, which happenedon the 9th of February, 1811, having occupied the situation of AstronomerRoyal for a period of forty-six years.

It is to Maskelyne that the original establishment of the “ NauticalAlmanac” is due. In 1763 he published a work, entitled “ The British

* In noting the passage of a star over the wires in the transit, Bradley in general tookthe nearest second, except when the interval was perceptible, in which case he marked theobservation * or —, to indicate that the true time exceeded or fell short of the time recorded.In some cases the time of transit is estimated in terms of the half or third of a second.Maskelyne, in the beginning of his career, divided the second into eight parts. In Sep-tember, 1772, he first commenced the practice of recording the times of transit to tenthsof a second .—(Miscellaneous Works of Bradley, p. liv.)