The tube is counterpoised as regards the polar axis by the weight a, and as regards the declination axis by the weights b b. At A, the upper section of the pier can be set in exact azimuth by adjusting screws, and at the base of the lower section the screws at B. B. allow some adjustment in latitude. To such mere rudiments are the azimuth and altitude circles of Short’s mount reduced. At the upper end of the polar axis is fitted the gear wheel g, driven by a worm from the clockwork at C to follow the stars in their course. At the lower end of the same axis is the hour circle h, graduated for right ascension, and a hand wheel for quick adjustment in R. A. At d is the declination circle, which is read, and set, by the telescope t with a right angled prism at its upper end, which saves the observer from leaving the eye piece for small changes. F is the usual finder, which should be applied to every telescope of 3 inches aperture and above. It should be of low power, with the largest practicable field, and has commonly an aperture ¼ or ⅕ that of the main objective, big enough to pick up readily objects to be examined and by its coarse cross wires to bring them neatly into the field. At m and n are the clamping screws for the right ascension and declination axes respectively, while o and p control the respective tangent screws for fine adjustment in R. A. and Dec. after the axes are clamped. This mount has really all the mechanical refinements needed in much larger instruments and represents the class of permanently mounted telescopes used in a fixed observatory. The ordinary small telescope is provided with a mount of the same general type but much simpler and, since it is not in a fixed observatory, has more liberal adjustments in azimuth and altitude to provide for changes of location. Figure 76 shows in some detail the admirable portable equatorial mounting used by the Clarks for instruments up to about 5 or 6 inches aperture. Five inches is practically the dividing line between portable and fixed telescopes. In fact a 5 inch telescope of standard construction with equatorial mounting is actually too heavy for practical portability on a tripod stand. The Clarks have turned out really portable instruments of this aperture, of relatively short focus and with aluminum tube fitted to the mounting standard for a 4 inch telescope, but the ordinary 5 inch equipment of the usual focal length deserves a permanent placement. In this mount the short tapered polar axis P is socketed between the cheeks A, and tightened in any required position by the hand screws B. The stout declination axis D bears the telescope and the counterweight C. Setting circles in R. A. and Dec., p and d respectively, are carried on the two axes, and each axis has a worm wheel and tangent screw operated by a universal joint to give the necessary slow motion. [Illustration: FIG. 76.—Clark Adjustable Equatorial Mount.]