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111 the tube of the telescope, or it may be fixed by glewing the tube into it as is done in mine. Having described the mechanism of this most simple, but accurate instrument, I shall point out a few of the uses and advantages this micrometer has over any other I have yet made use of, or seen described. [[left margin]] Planets, their diameters observed.[[/left margin]] 1.^0 In observing the planets diameters, it is well know to such as have been used to Kirchius's micrometer (of two screws moving in a ring and meeting in the center of the field of view) the observations are momentary; but with mine it will be found quite otherwise, for having adjusted the hairs nearly to the planete's Diameter, and turned the tube with the michrometer in such a manner that the planet may pass exactly between, just touching each hair thro' the whole field of the telescope, the observation may be improved, or corrected, for the space of a minute and an half, or two minutes, and consequently the diameter taken with the utmost ease and exactness. [[Left margin]] Occultations, an advantage in observing them. [[/left margin]] 2.^0 In the occultations of fixed stars, &c. if one of the hairs in my micrometer be made to bisect the moon at right angles to the cusps, or nearly so, and the other to touch the star near the point of immersion, it will also cut the other edge of her disk in the point of emersion; for want of knowing which, the observer may miss the moment of emersion. See fig. 14, and 15. where [[bold]] ab [[bold]] is the bisecting line, c the point of immersion, and [[underline]] d [[/underline]] that of emersion. [[left margin]] Planets, their R. A. & Decl.^n observed. [[/left margin]] 3.^0 When any of the [[strikeout]]known[[/strikeout]]planets are so near to a known [[insert mark]][[superscript]]fixed star[[/]] as to be both within the field of the telescope, the difference of right ascensions and declinations are found thus, turn the micrometer and adjust the screws in such a manner that the star moving along one hair, the planet may move along the other, then will the micrometer shew their difference of declinations, and turning the tube of the micrometer 1/4 round, and adjusting the hairs so that when the star touches one, the planet may touch the other hair, then will the micrometer shew the difference of right ascentions, see fig. 16. in which [[bold]]a*[[/bold]] and [[symbol for planet]][[bold]b[[/bold]] shew the position of the hairs in the difference of the declinations; and [[bold]]c*[[/bold]] and [[symbol for planet]][[bold]]d[[/bold]] in the difference of right ascentions. Note, this is the true difference of right ascention if the planet be in or very near the equinoctial, otherwise must be increased in proportion of the co-sime of the planet's declination to the radius. -- I might have added more, but room will not permit I shall only just mention, that these micrometers may be easily applied to the reflecting telescope, and observations made with the same ease and exactness. From (Martin's) the General Magazine, Sept.[[superscript r?]] 1764. p. 440. [[left margin]]Effects of the precession of the Equinoxes. V.p. 19 where 2 stars are calculated to shew this. [[/left margin]] By means of the precession of the Equinoctial points [[strikethrough]]the[[/strikethrough]] in [[underline]]Antecedentia[[/underline]] (or from East to West, contrary to the order of the signs)50'' each year, the fixed Stars appear to move as much in [[underline]] Consequentia [[/underline]] (or from West to East, in the order of the signs) increasing their longitudes 50'' every year whilst their Latitudes always remain the same. Thereby they likewise continually increase their Right Ascension,[[insert mark]] [[superscript]]and[[/superscript]] [[strikethrough]] declination; For Stars having [[strikethrough]]a[[/strikethrough]][[underline]]North[[/underline]] declination & their Right Ascension between the Solsticial Colure of [[underline]]Capricor [[insert mark]][[superscript]]n[[/superscript]][[/underline] & [[underline]]Cancer[[/underline]]; Also those having [[underline]]South[[/underline]] declination & their Right Ascension between the Solsticial Colure of [[underline]]Cancer[[/underline]] and [[underline]]Capricorn[[/underline]] --- [[right margin note following right-facing brace]] [[underline]]Increase[[/underline]] their Declination. But, Stars having [[underline]]S.[[/underline]] Decl. & their R.A. between the Solsticial Colure of [[underline]]Capricorn[[/underline]] and [[underline]]Cancer[[/underline]]; Also, those having [[underline]]N.[[/underline]] Decl & their R.A. between the Solsticial Colure of [[underline]]Cancer[[/underline]] and [[underline]]Capricorn[[/underline]] -- [[Right margin note following right-facing brace]] Decrease their Declination [[page end]]
