IN the process of measuring the places of stars on the celestial sphere, or in the converse process of using these measured places to fix the position of the observer upon the earths surface, the astronomer has at his disposal two systems of reference lines or circles upon which to base his measurements. These are respectively the vertical great circles through his zenith and the small circles parallel to his horizon, the circles of equal altitude or equal zenith distance. Using the first system, his method is to time the transit of a star across a vertical circle, almost invariably the meridian circle passing through the north and south points. If, in addition to timing the transit, he measures the altitude, he gets a complete determination of the position of the star observed, and uses both sets of reference circles, the vertical circle for fixing the time of transit, and hence the right ascension of the star, and the horizontal circle for fixing the altitude of transit, and hence the star's declination. This is the ordinary observation carried out in the observatory with the transit circle or by the surveyor in the field with the theodolite. Another method of observation which gives the same quantities, though not in the same direct form, is by the use of an instrument adapted for the recording of transits across a horizontal circle of constant altitude. An instrument of this class is the almucantar, in which horizontality is secured by the device of floating the whole in a mercury bath, it being easily seen that if either the instrument or the bath is moved round, the telescope will maintain a constant angle with the horizontal, and the line of vision will therefore always intersect an almucantar or circle of equal altitude.