PARALLACTIC INSTRUMENT, or Ptolemy's Rules, the name given to an instrument invented by Ptolemy for determining the moon's parallax, and described by him, Almagest,' b. v., c. 12.
A B, ? c, dig. 1, are two stout wooden rods turning on a pin or centre at a ; s a=nc; ac is a third divided rod, also turning on a pin et A, and passing through a loop or bifurcation of the rod )3c at c ; r q, a plumb-line by which A B is adjusted vertically ; E F, two sights fixed on c a. It is evident, if A B be truly perpendicular, and any object be morn in the direction a r, that A c will be the chord of the angle A Re, that is, of the zenith distance of the object. A c may be divided as a scale of equal parts, and the angle deduced from a table of chords, or as a line of chords to radius A a, In which case the angle may be ;end off at once.' X figure of a similar instrument may bo found In Tycho Bralle, Astron. Inetaur. Mechanics,' sheet c. We have thought Ptokmy's Rules worth notice chiefly as pointing out a very cheap and accurate instrument for obtaining the time by the method of equal altitudes. Let A n be a rod turning on pivots above and below,—for instance, between the sill and architrave of a window,—and capable of being set perpendicular by a plumb.line, supposed in the figure to hang behind A B. E C, A stout edge-bar turning round a pin at o, and fixable in any direction by two laths, E M, c N. On E c a telescope is lashed (one sufficient for the purpose might easily be made of a tin tube and spectacle-glasses), with one or more horizontal wires placed in the focus. If the times of contact of the sun's limbs with the hori
zontal wires be observed in the forenoon and again in the afternoon (the axis A II being in both cases truly adjusted, and the instrument in other respects unchanged), the data will be obtained for finding the time shown by the watch when the sun is on the meridian—that is, apparent noon—with great accuracy and very little calculation. (See Schumacher's' Hiilfstafeln;p. 49, &c. ; Bailer's Tables,' pp. 92, 148, and many other collections.) The determination of the error of the watch by equal altitudes of the same star is even more simple, as there is then no motion in declination to be allowed for, and the middle time by the watch of similar observations is the time when the star passes the meridian, or its right ascension, which is known from the Nautical Almanac,' or from well-known catalogues. The observer must bo very careful to adjust the instrument by the plumb-line to the same position in both observations, and he must have a watch which will keep time correctly for the interval.
Sometimes the word parallaetique is used to signify the instrument known to us by the name of EQUATORIAL. Leland° (' Astronomic,' § 2278) says the proper word is parallatique, or that which follows the parallel of stars; be reserves paralleetique for Ptolemy's Rules.