Another form, even simpler and found to be extremely satisfactory even for rather large instruments, is the open polar fork mount. Here the polar axis of an ordinary form is continued by a wide and stiff casting in the form of a fork within which the tube is carried on substantial trunnions, giving it complete freedom of motion. The open fork mount in its simplest form, carrying a heliostat mirror, is shown in Fig. 81. Here _A_ is the fork, _B_ the polar axis, carried on an adjustable sector for variation in latitude, _C_ the declination axis carrying the mirror _D_ in its cell, _E_ the slow motion in declination, and _F_ that in R. A. Both axes can be unclamped for quick motion and the R. A. axis can readily be driven by clock or electric motor. The resemblance to the fully developed English equatorial mount of Fig. 78 is obvious, but the present arrangement gives entirely free swing to a short instrument, is conveniently adjustable, and altogether workmanlike. It can easily carry a short focus celestial camera up to 6 or 8 inches aperture or a reflector of 4 or 5 feet focal length. In Fig. 173, Chap. X a pair of these same mounts are shown at Harvard Observatory. The nearer one, carrying a celestial camera, is exposed to view. It is provided with a slow motion and clamp in declination, and with an electric drive in R. A., quickly unclamped for swinging the camera. It works very smoothly, its weight is taken by a very simple self adjusting thrust bearing at the lower end of the polar axis, and altogether it is about the simplest and cheapest equatorial mount of first class quality that can be devised for carrying instruments of moderate length. Several others are in use at the Harvard Observatory and very similar ones of a larger growth carry the 24 inch Newtonian reflector there used for stellar photography and the 16 inch Metcalf photographic doublet. [Illustration: FIG. 82.—Mounting of Mt. Wilson 60-inch Reflector.] [Illustration: FIG. 83.—The 60-inch as Cassegrainian, F = 100′.] In fact the open fork mount, which was developed by the late Dr. Common, is very well suited to the mounting of big reflectors. It was first adapted by him to his 3 ft. reflector and later used for his two 5 ft. mirrors, and more recently for the 5 ft. instrument at Mt. Wilson, and a good many others of recent make. Dr. Common in order to secure the easiest possible motion in R. A. devised the plan of floating most of the weight assumed by the polar axis in mercury. Figure 82 is, diagrammatically, this fork mount as worked out by Ritchey for the 5′ Mt. Wilson reflector. Here A is the lattice tube, B the polar axis, C the fork and D the hollow steel drum which floats the axis in the mercury trough E. The great mirror is here shown worked as a simple Newtonian of 25 ft. focal length. As a matter of fact it is used much of the time as a Cassegranian.