[Illustration: Fig. 28. Sectional View of Observatory.] The iron wire rope required to be carefully made, so as to avoid rigidity. It contains 2-1/3 miles of wire, 1/100 of an inch in diameter, and has 300 strands. Each single wire will support 7 pounds. It is, however, more flexible than a hempen rope of the same size, owing to its loose twisting. [Illustration: Fig. 29. The Mirror Support.] At the lower end of the tube, at the distance of a foot, and crossing it at right angles, held by three bars of iron _i i′ i″_, Fig. 29, is a circular table of oak _e_, which carries an India-rubber air sac _d_, and upon this the mirror _f_ is placed. The edge support of the mirror is furnished by a semicircular band of tin-plate _a_, lined inside with cotton, and fastened at the ends by links of chain _b_, (_b′_ not seen) to two screws _c c′_; _g_ and _h_ are the wire ropes, marked _b_ and _h_ in Fig. 28. Instead of the blanket support which Herschel found so advantageous, M. Foucault has suggested this use of an air sac. In his instrument there is a tube going up to the observer, by which he may adjust its degree of inflation. It requires that there should be three bearings _c c′ c″_, in front of the mirror, against which it may press when the sac behind is inflated, otherwise the optical axis is altogether too instable, and objects cannot be found. The arrangement certainly gives beautiful definition, bringing stars to a disk when the glass just floats, without touching its front bearings. The first sac that I made was composed of two circular sheets of India-rubber cloth, joined around the edges. But this could not be used while photographing, because the image was kept in a state of continuous oscillation if there was a breeze, and even under more favorable circumstances took a long time to come to rest. It was not advisable to blow the mirror hard up against its three front bearings, in order to avoid the instability, for then every point in of an object became triple. To the eye the oscillations were not offensive, because the swaying image was sharp. Subsequently, however, an air chair cushion was procured, and as the surface was flat instead of convex the difficulty became so much less, that the blanket support was definitely abandoned. It is necessary that the mirror should have free play in the direction of the length of the tube when this kind of support is used, and that is the reason why the tin edge hoop must terminate in links of chain. The interval, eight or ten inches, which separates the face of the mirror from the tube, is occupied by a curtain of black velvet, confined below by a drawing cord and tacked above to the tube. This permits access to the mirror to put a glass cover on it, and when shut down stops the current of air rushing up. When the instrument is not being used this curtain is left open, because the mirror and tube are in that case kept more uniform in temperature with the surrounding air.