The sliding roof type is on the whole the simplest structure that can be regarded as an observatory in the sense of giving some shelter to the observer as well as the instrument. It gives ample sky room for practical purposes even to an instrument with a fork mount, since in most localities the seeing within 30° or so of the horizon is decidedly bad. If view nearer the horizon is needed it can readily be secured by building up the pier a bit. Numberless modifications of the sliding roof type will suggest themselves on a little study. One rather interesting one is used in the housing of the 24 inch reflector of the Harvard Observatory, 11 feet 3 inches in focal length, the same of which the drive in its original dome is shown in Fig. 139. As now arranged the lower part of the observatory remains while the upper works are quite similar in principle to the housing of Dr. Common’s 3 foot reflector of Fig. 176. The cover open is shown in Fig. 178. It will be seen that on the north side of the observatory there is an out-rigger on which the top housing slides clear of the low revolving turret which gives access to the ocular fitting used generally to carry the plate holder, and the eyepiece for following when required. The tube cannot be brought to the horizontal, but it easily commands all the sky-space that can advantageously be used in this situation, and the protection given the telescope when not in use is very complete. To close the observatory the tube is brought north and south and turned low and the sliding roof is then run back into its fixed position. The turret is very easily turned by hand. [Illustration: FIG. 178.—Turret Housing of the 24-inch Harvard Reflector.] Of course for steady work with the maximum shelter for observer obtainable without turning to highly special types of housing, the familiar dome is the astronomer’s main reliance. It is in the larger sizes usually framed in steel and covered with wood, externally sheathed in copper or steel. Sometimes in smaller domes felt covered with rubberoid serves a good purpose, and painted canvas is now and then used, with wooden framing. But even the smallest dome of conventional construction is heavy and rather expensive, and for home talent offers many difficulties, especially with respect to the shutter and shutter opening. A hemisphere is neither easy to frame nor to cover, and the curved sliding shutter is especially troublesome. [Illustration: FIG. 179.—The Original “Romsey” Observatory.]