When the equatorial is once properly in adjustment, not only can the co-ordinates of a celestial body be observed with accuracy when the time is known, but a planet or other body can easily be found in the day-time. The object is found by the two circles—the declination circle and the hour or right-ascension circle. The declination of the required object being given, the telescope is set by the circle to the proper angle with the equator. The R.A. of the object is then subtracted from the sidereal time, or that time plus twenty-four hours, which will give the distance of the object from the meridian, and to this distance the hour-circle is set. The object should then be in the field of the telescope, or at least in that of the finder. We subtract the star’s R.A. from the sidereal time because the clock shows the time since the first point of Aries passed the meridian, and the star passes the meridian later by just its R.A., so that if the time is 2_h._, or the first point of Aries has passed 2_h._ ago, a star of 1_h._ R.A., or transiting 1_h._ after that point, will have passed the meridian 2_h._ - 1_h._ = 1_h._ ago; so if we set the telescope 1_h._ west of the meridian we shall find the star. The moment the object is found the telescope is clamped in declination, and the clock thrown into gear, so that the star may be followed and observed for any length of time. ----- Footnote 18: The altitude of the star in this case is its declination plus the co-latitude of the place, but this only applies when the star is on the meridian. When the altitude of a star in another situation is required, it is found sufficiently accurately by means of a globe. A sextant, if at hand, will of course give it at once. Footnote 19: Since the velocity of the star varies as the cosine of the declination, the error of collimation at the equator = 2m. 25·5s. cos. 0° 45´·5 = 2m. 25·08s.; and for non-equatorial stars, 2m. 25·08s. sec. dec. Footnote 20: This error varies as the tangent of the declination, and therefore to find the constant for the instrument, in case the parts do not admit of easy adjustment, we divide 4m. 1s. by 1·18 the tan. of Dec. of η Ursæ Majoris, giving 3 min. 28 sec. CHAPTER XXII. THE EQUATORIAL OBSERVATORY.