AstronomyDetermination of time, longitude, latitude, and azimuth . tration Xo. 13 shows one of the best zenith telescopes now in use hi tins Survey. Thisinstrument, Zenith Telescope No. 4, was originally made by Troughton & Simins, of London,in 1849, and was remodeled at the Coast and Geodetic Survey Office in 1891. It carries atelescope with a clear aperture of about 76mm (3 inches), and a focal length of about 116,6cm(46 inches). The magnifying power with the eyepiece ordinarily used is 100 diameters. Twolatitude levels are used instead of one, to secure increased accuracy. Each of these lev
AstronomyDetermination of time, longitude, latitude, and azimuth . tration Xo. 13 shows one of the best zenith telescopes now in use hi tins Survey. Thisinstrument, Zenith Telescope No. 4, was originally made by Troughton & Simins, of London,in 1849, and was remodeled at the Coast and Geodetic Survey Office in 1891. It carries atelescope with a clear aperture of about 76mm (3 inches), and a focal length of about 116,6cm(46 inches). The magnifying power with the eyepiece ordinarily used is 100 diameters. Twolatitude levels are used instead of one, to secure increased accuracy. Each of these levelscarries a graduation which is numbered continuously from one end to the other (instead ofeach way from the middle), the numbering of the upper one running from 0 to 50 and of thelower from 60 to 110. A 2mm division on the upper level has a value of about and on thelower about 1,4. The vertical axis of the instrument is in the vertical plane in which thetelescope swings. The clamp arm, perforated for the sake of lightness, gives the telescope a No. ZENITH TELESCOPE. ii DETERMINATION OF LATITUDE. 105 marked degree of stability in so far as changes of inclination are concerned, The eyepiecemicrometer, arranged to measure zenith distance, has a value of about 45 per turn, and themicrometer head is graduated to hundredths of a turn. The better known type of zenith telescope, in which the telescope is mounted eccentricallyon one side of the vertical axis instead of in front of it, is also in use in the Survey. The meridiantelescopes described on page 8 are extensively used for latitude determinations, as well asfor time. In latitude work with the meridian circle at astronomic observatories the instrument isusually fitted with a reversing prism. By rotating this prism the apparent motion of the staris changed from the direction right to left to the direction left to right or vice versa. A pointingis made on the star before it transits, the prism is reversed, and a second p
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