The Altazimuth mounting, from its simplicity and comparative cheapness, has all along been, and will probably continue to be, the form most used by amateurs. It is, however, decidedly inferior in every respect to the equatorial form of mount. In this form (Fig. 11) the telescope is carried by means of two axes, one of which--the Polar axis--is so adjusted as to be parallel to the pole of the earth's rotation, its degree of inclination being therefore dependent upon the latitude of the place for which it is designed. At the equator it will be horizontal, will lie at an angle of 45 degrees half-way between the equator and either pole, and will be vertical at the poles. At its upper end it carries a cross-head with bearings through which there passes another axis at right angles to the first (the declination axis). Both these axes are free to rotate in their respective bearings, and thus the telescope is capable of two motions, one of which--that of the declination axis--enables the instrument to be set to the elevation of the object to be observed, while the other--that of the polar axis--enables the observer to follow the object, when found, from its rising to its setting by means of a single movement, the telescope sweeping out circles on the sky corresponding to those which the stars themselves describe in their journey across the heavens. This single movement may be given by means of a tangent screw such as has already been described, and the use of a telescope thus equipped is certainly much easier and more convenient than that of an Altazimuth, where two motions have constantly to be imparted. To gain the full advantage of the equatorial form of mounting, the polar axis must be placed exactly in the North and South line, and unless the mounting can be adjusted properly and left in adjustment, it is robbed of much of its superiority. For large fixed instruments it is, of course, almost universally used; and in observatories the motion in Right Ascension, as it is called, which follows the star across the sky, is communicated to the driving-wheel of the polar axis by means of a clock which turns the rod carrying the tangent screw (Plate II.). These are matters which in most circumstances are outside the sphere of the amateur; it may be interesting for him, however, to see examples of the way in which large instruments are mounted. The frontispiece, accordingly, shows the largest and most perfect instrument at present in existence, while Plate II., with Figs. 4 and 12, give further examples of fine modern work. The student can scarcely fail to be struck by the extreme solidity of the modern mountings, and by the way in which all the mechanical parts of the instrument are so contrived as to give the greatest convenience and ease in working. Comparing, for instance, Plate II., a 6-inch refractor by Messrs. Cooke, of York, available either for visual or photographic work, with the Dorpat refractor (Fig. 3), it is seen that