VOL. XLVI.] PHILOSOPHICAL TRANSACTIONS. 6Q7 



it will not differ at any time from the true meridian above 10' of time; and if a 

 proper allowance be made for the refraction at the time of observation, it may 

 be found much more exact. This line thus found will be of use to save trouble 

 afterwards; and is indeed the foundation of all astronomical observations. 



To find a Star or Planet in the Day-time, even at Noon-day. — ^The instrument 

 remaining as rectified in the last experiment, set the declination semicircle to the 

 declination of the star or planet to be seen ; and then set the equatorial plate to 

 the right ascension of the star or planet at that time; and, looking through the 

 telescope, the star or planet will be seen ; and after having once got it into the 

 field, you cannot lose it; for, as the diurnal motion of a star is parallel to the 

 equator, by moving the equatorial handle so as to follow it, you will at any time, 

 while it is above the horizon, recover it, if it be gone out of the field. 



The easiest method for seeing a star or planet in the day-time is this: the in- 

 strument being adjusted as before-directed, bring the telescope down so as to 

 look directly at the meridian mark ; and then set it to the declination, and right 

 ascension, as before-mentioned. 



By this instrument, most of the stars of the first and second magnitude have 

 been seen even at mid-day, and the sun shining bright; as also Mercury, Venus, 

 and Jupiter; Saturn and Mars are not so easy to be seen, on account of the 

 faintness of their light, except when the sun is but a few hours above the 

 horizon. 



And in the same manner in the night-time, when you can see a star, planet, 

 or any new phenomenon, such as a comet, you may find its declination and right 

 ascension immediately, by turning about the equatorial handle, and declination 

 handle, till you see the star, planet, or phenomenon : and then, looking on the 

 equatorial plate, you find its right ascension in time ; and you find, on the decli- • 

 nation semicircle, its declination in degrees and minutes. 



In order" to have the other uses of this instrument, make the equatorial plates 

 become parallel to the horizontal plates; and then this instrument becomes an 

 equal altitude instrument, a transit instrument, a theodolite, a quadrant, an azi- 

 muth instrument, and a level. The manner of applying it to these different 

 purposes is too obvious to need any explanation. 



As there is also a box with a magnetic needle fastened in the lower plate of 

 this instrument, by it you may adjust the instrument nearly in the meridian; and 

 by it also you may find the variation of the needle; if you set the horizontal 

 meridian, and the equatorial meridian, in the middle of their nonius indexes, 

 and direct the telescope to the meridian mark, observe how many degrees from 

 the meridian of the box the needle points at; and this distance or difi'erence is 

 the variation of the needle. 



VOL. IX. 4 U 



