Now what does the modern astronomer do with this instrument when he has got it? It is absolutely without circles, but the faithful companion of the Transit Instrument is the Astronomical Clock—and the two together serve the purpose of a circle of the most perfect accuracy, so that by means of these two instruments we shall be able to determine the right ascensions of all the stars merely by noting the time at which the earth’s rotation brings them into the field of view. The clock having been regulated to sidereal time, a term fully explained in the sequel, it will show 0_h._ 0_m._ 0_s._ when the first point of Aries passes the meridian, and instead of dividing the day into two periods of twelve hours each, the clock goes up to twenty-four hours. If now a star is observed to pass the centre of the field of view (that is the meridian) at 1_h._ by the clock, or one hour after the first point of Aries, it will be known to be in 1_h._ of right ascension; or if it passes at 12_h._ it will be 12_h._ right ascension, or opposite to the first point of Aries, and so on up to the twenty-four hours, the clock keeping exact time with the earth. The transit instrument thus gives us the right ascension of a star, or one co-ordinate: and now we want the other—the declination. THE TRANSIT CIRCLE. This is given by the Transit Circle, which is a transit instrument with a circle attached, to ascertain the angle between the object and the pole or equator. [Illustration: FIG. 116.—The Mural Circle. ] The combination of the circle with the transit, forming the transit- or meridian-circle, is of comparatively recent date, and the earlier method was to use a circle with a telescope attached, fixed to a pivot moving on bearings in a wall, and called therefore the Mural Circle, Fig. 116. Since it is supported only on one side it cannot move so truly in the meridian as the transit, but, having a large circle, it gives accurate readings. [Illustration: FIG. 117.—Transit Circle, showing the addition of circles to the transit instrument. ] [Illustration: FIG. 118.—Perspective view of Greenwich Transit Circle. ]