In spite of such contrivances there is still sometimes a strong residual current in the tube. I have tried to overcome it by covering the mouth of the tube with a sheet of flat glass, but have been obliged to abandon that because the images were injured. At one time, too, when it was supposed that the current was partly from the observer’s body, heated streams of air going out around the tube, the aperture in the dome was closed by a conical bag of muslin, which fitted the mouth of the telescope tightly. The only advantages resulting were mere bodily comfort and a capability of perceiving fainter objects than before, because the sky-light was shut off. [Illustration: Fig. 30. Section of Azimuth Axis.] b. _The Supporting Frame._ The frame which carries the preceding parts is of wood, and rests on a vertical axis _a_, Fig. 30, turning below in a gun-metal cup _b_, supported by a marble block resting on the solid rock. The upper end of the axis is sustained by two collars, one _c c′_ above, and the other below an intermediate triangular box _e e′_ from the sides of which three long beams _f f f_ 12 × 3 inches diverge, gradually declining till they meet the solid rock at the limits of the excavation in which the observatory is placed. These beams are fastened together by cross-pieces _g g g_, Fig. 31, and go through the floor in spaces _h h h_, so contrived that the floor does not touch them. At the ends they are cased with a thick leaden sheathing, to deaden vibration and prevent the access of moisture. [Illustration: Fig. 31. Plan of Observatory (lower floor).] This tripod support in connection with the sustaining of the telescope by the wire rope, gives that steadiness which is so essential in photography. Only a slight amount of force, about two pounds, is required to move the instrument in azimuth, though it weighs almost a thousand pounds. The plan of the frame centrally carried by the axis _a_ is as follows: From the corners of a parallelogram _i i_ (2 × 13 feet) of wooden beams, eight inches thick and three inches broad, perpendiculars _n n′_, Fig. 28, rise. At the top they are connected by lighter pieces to form a parallelogram, similar to that below, and just large enough to contain the tube of the telescope. At right angles to the parallelogram below, and close upon it, a braced bar _o o′_, Fig. 28, crosses. From its extremities four slanting braces as at _p p′_, Fig. 28, go to the corners of the upper parallelogram, and combine to give it lateral support. At the top of one close pair of the perpendiculars _n′_, Fig. 28, are bronze frames carrying friction rollers upon which the trunnions move, while similarly upon the other pair _n_ are two pulleys, also on friction rollers, for the wire rope coming from the counterpoises.