VOL. LVIIl.J PHILOSOPHICAL TRANSACTIONS. 543 



constantly supposed the distance of the common centre of gravity from the 

 centre of the earth, to be a fixed quantity ; whereas it will vary in the same pro- 

 portion as the moon's distance varies ; but, as this and many other minutiae will 

 properly enter the computation, when the observations are made, he refers them 

 to the learned in this science. 



XXV. Description of a New Method of Observing the Heavenly Bodies out of 

 the Meridian. By J. Smealon, F. R. S. p. 170. 



The instrument Mr. S. proposes for this purpose, is a transit telescope, 

 mounted on a vertical axis ; for example, such a one as is described in the intro- 

 duction to the Histoire Celeste of Mr. le Monnier ; being one of the instru- 

 ments made by Mr. Graham for the academicians who went to measure a degree 

 at the Polar circle ; this, or any other instrument on equivalent principles, will 

 suffice, that is capable of such adjustments, as to be made correctly to describe 

 an almacanther and azimuth circle ; and capable of being retained in any given 

 position ; the use will appear by the following example. 



Make choice of any fixed star, which, according to the diurnal motion, pre- 

 cedes the heavenly body to be observed by a few minutes, more or less, as it may 

 happen ; let the instrument be set to an azimuth, somewhat preceding the fixed 

 star ; and carefully observe the time of the star's transit across the vertical 

 wire of the telescope ; then wait till the heavenly body comes to the same 

 azimuth ; and, when arrived within the field of view, keep gently turning the 

 screw that alters the elevation of the telescope, so as to follow the heavenly body 

 in altitude ; keeping it intersected by the horizontal wire of the telescope, till 

 the body passes the middle vertical wire, and carefully note the time of its passage; 

 there leave the telescope fixed as to altitude, and releasing the horizontal motion, 

 turn it round on its vertical axis, till you meet with some star, that in a little 

 time after will, by rising or falling, come to the same almacanther ; and on its 

 arrival, carefully note the time of its passage across the horizontal hair of the 

 telescope. 



Now, from the right ascensions and declinations of the two stars being pre- 

 viously known, or afterwards determined from meridian observations ; the azi- 

 muth of the first star, and the altitude of the last, at the time of their respec- 

 tive passages, may be determined by computation ; which will give the altitude 

 and azimuth of the heavenly body, for the time of the middle observation, when 

 it passed the intersection of the two wires. 



The same end may also be obtained by taking the observations in an inverted 

 order ; that is, by chusing a star at such an altitude, that the heavenly body 

 shall in a competent time afterwards arrive at the same altitude, &c. but, as in 

 these latitudes the alteration of azimuth is, especially in those parts that are in 



