The members _U_ constitute a framing which supports at once the housing and the observing platform, to which access is had by a ladder, _Z_, provided with a counterbalanced observing seat. The instrument is put into action by clearing the door at the end of the roof, running the roof back to the position shown in the dotted lines, raising the tube, and then revolving the whole housing into whatever position is necessary to permit the proper setting of the tube. [Illustration: FIG. 177.—Sliding Roof Observatory.] This arrangement worked well but was found a bit troublesome owing to wind and weather. With a skeleton tube and in a favorable climate the plan would succeed admirably providing an excellent shelter for a large telescope at very low cost. Since a fork mount allows the tube to lie flat, such an instrument, up to say 8 or 10 inches aperture can be excellently protected by covers fitting snugly upon a base and light enough to lift off as a whole. The successful use of all these shelters however depends on climatic conditions. They require circumstances allowing observation in the open, as with tripod mounts, and afford no protection from wind or cold. Complete protection for the observer cannot be had, except by some of the devices shown in Chapter V, but conditions can be improved by permanent placement in an observatory, simple or elaborate, as the builder may wish. The word observatory may sound formidable, but a modest one can be provided at less expense than a garage for the humblest motor car. The chief difference in the economic situation is that not even the most derided car can be picked up and carried into the back hall for shelter, and it really ought not to be left out in the weather. The next stage of evolution is the telescope house with a sliding roof in one or more sections—ordinarily two. In this case the building itself is a simple square structure large enough to accommodate the instrument with maneuvering room around it. The side walls are carried merely high enough to give clearance to the tube when turned nearly flat and to give head room to the observer. The roof laps with a close joint in the middle and each half rolls on a track supported beyond the ends of the building by an out-rigger arranged in any convenient manner. When the telescope is in use the roof sections are displaced enough to give an ample clear space for observing, often wide open as shown in Fig. 177, which is the house of the 16 inch Metcalf photographic doublet at the Harvard Observatory. This instrument is in an open fork mount like that shown in Fig. 139.