370 ANNUAL OF SCIENTIFIC DISCOVERY. 



GREENWICH OBSERVATORY. 



FROM the official report of the Superintendent of Greenwich 

 Observatory, Prof. Airy, we derive the following particulars : The 

 long established system of meridional observations, for which Green- 

 wich has been so long and so justly celebrated, is maintained inviolate. 

 Each standard star is observed, if possible, twenty times in three 

 years. The moon is always observed on the meridian when visible, 

 and the sun and planets, except on Sundays. The number of obser- 

 vations from 1851, May 30, to 1852, May 18, was, transits, 4,500 ; circle 

 observations, 5,000. Under the head, " Reduction of Observations," 

 Mr. Airy states that the reduction of a long series of observations of 

 the solar spots is much advanced, and that the occultations of 1851, and 

 the double image micrometer measures to the present time, are com- 

 pletely reduced. With respect to the reduction of the observations 

 made by the transit circle, a curious circumstance is mentioned. 

 While the construction of this instrument, and the modes of observa- 

 tion with it, have given a warranty, such as the world never possessed 

 before*, for the steadiness of the instrument and its adjuncts, there 

 have been instances when the azimuth of the instrument, greatly to 

 the surprise of the Astronomer Royal, has varied four seconds, as de- 

 termined by opposite passages of the pole star. Mr. Airy has no 

 other way of explaining this than the supposition that the " sound and 

 firm-set earth " itself is in motion. 



A new reflex zenith telescope has been introdced. The fundamental 

 principle of its construction is, that the micrometer and wire-frame 

 are carried by the frame of the object-glass, and that the convergent 

 pencil of light coming from a star is received, at a distance of half its 

 focal length, upon the surface of quicksilver contained in a trough, 

 whose support is independent of the telescope . 



OX DETERMINING LONGITUDE AT SEA FROM THE ALTITUDE OF 



THE MOON. 



THE following paper was read before the Royal Astronomical Soci- 

 ety, England, by Lt. Ashe, R. N. Lt. A. states, that during an inter- 

 val of leisure in 1849, his attention was drawn to the subject of 

 longitudes, by the circumstance of one of the chronometers (which 

 had gone well for four years) changing its rate very considerably 

 without any assignable cause : and knowing what implicit confidence 

 is placed in a good chronometer, I felt that it was to be lamented that 

 there was not some plan less laborious and of easier attainment than 

 the lunar distance for checking chronometers ; for, notwithstanding 

 my experience of more than twenty years' sea time, I can only recol- 

 lect one instance of the chronometers having been checked by the 

 lunar distance, which may be, perhaps, accounted for by the many 

 inconveniences attending the observation, and the large amount of 

 practice necessary to ensure success. The data for the problem 

 proposed by Lieut. Ashe, are, the sidereal time of the observation, the 



