414 Additional Notes. 



ther to add, that the use of the instrument is very easy. For 

 if the index be set so near the distance of the moon and stars, 

 and the Hmb so held as to cut the body of the moon, upon 

 directing the telescope to the star, her image will, of course, 

 be reflected on some part of the speculum G H. There is 

 no. absolute necessity the star and moon should coincide ex- 

 actly at the line limiting the silvered and unsilvered part of 

 the latter speculum ; for the transparent part of that glass will 

 often reflect the moon's image sufliciently for the telescope 

 to take it, artd if her limb in that and the star exactly coin- 

 cide near it, it may be suflScient, though the nearer to' that 

 line the better. Now their distance being found, the tables 

 that give the moon's place may be depended on for iier dia- 

 meter and her latitude, which last being known, there are 

 three sides of a triangle given to find the angle at the pole of 

 the ecliptic, which, compared with the star's longitude, de- 

 termines her place for that instant : for, in respect to her la- 

 titude when she is swiftest in motion, when nearest her nodes^ 

 and when the inclination of the orbs is greatest (if these could 

 all happen together), yet the variation of her latitude, in the 

 space of one hour, equal to 1 5 deg. of longitude on the earth, 

 if a star be taken somewhat near the ecliptic, and not very 

 near the moon, will not alter the angle at the pole but a very 

 few seconds. The nearness of the speculum G H is no dis- 

 advantage, because the rays come reflected in the same man- 

 ner as they come direct. It may be needless to add that, 

 when practicable, the moon should be taken when near the 

 meridian — or that the instrument will equally take the dis- 

 tance of the sun from the moon, when visible, as she often 

 is, in the day-time ; for which purpose there must be a place 

 made at M for a darkening glass, to be fixed there when ne- 

 cessary, and the telescope directed to the moon. Nor need I 

 add, that the same instrument will very well serve for taking 

 the distance of any two stars, a comet, bcc, always taking 

 the brightest by reflection; all which is obvious. But I mu&c 

 further observe, with pleasure, that if we do not quite mis- 

 take in all that has been said here, there is now a method 

 found by it to obtain what is equivalent to a bodily appulse of 

 the moon to a fixed star, or to the sun at any moment when 

 visible, which, indeed, might be wished; but if the longi- 

 tude could ever be expected to be determined by the mo- 

 tions of the moon (to which end J. Flamstead's, and thy 

 more assiduous labours in observing her, have, I suppose, 

 been principally levelled), and this instrument be duly made 



