Protecting Cover.—A spherical calotte of single silk, varnished with four coats and vaselined, and having an area of 1,560 square feet, covers the upper part of the balloon. It is finished in the same way as the balloon, being composed of overlapping widths to the number of 720.
The seams, having a width of ¼ inch, are hooked together, and sewn with two rows of stitching with silk thread. They are not covered with cemented strips. Their total length is 656 yards.
The vertex of the protecting cover, of conical shape, is of double silk. It rests on a small wooden structure, likewise of conical shape, the base of which, placed on the material at the upper pole of the balloon, is surrounded and held in place by the crown of the net.
The lower zone, terminating the protecting cover, is of double silk. Its extreme edge is put round a hemp bolt-rope, ³⁄₁₀ inch in diameter, which is sewn into the material; ninety-six eyes made above this bolt-rope are intended to receive thongs for fixing the protecting cover to the meshes of the net.
This protecting cover weighs 88 lbs. Its object is to protect the top of the balloon against rain, and more especially to prevent incrustation with snow between the meshes of the net.
Valves.—The balloon has no valve at the top, because its action would probably be affected by snow; but it is provided with two manipulating valves, of equal dimensions, one being placed at the equator, and the other one metre above the equator. These are at 150 degrees angular distance from each other.
These valves, constructed on a system of Andrée’s, measure 9⅘ inches in external diameter. Each is formed by a disc of aluminium bronze, 9 inches in diameter, and under normal conditions rests on a circular piece of walnut wood, but can be withdrawn therefrom by moving it along a screw which occupies the centre. This movement is effected from the car of the balloon by means of two cords for each valve, passing through the interior of the balloon and issuing from it near the appendage through four tubes, arranged in pairs.
Gas-tightness is obtained by a circular rubber band, against which the outer margin of the valve disc rests. The orifice for the escape of gas measures 7⅘ inches in diameter. In order to fix the valves to the balloon, two openings, 7⅘ inches in diameter, are made in the material. The margin around these openings is strengthened by collars, 19⅖ inches in diameter, of threefold material, cemented and sewn on; the seat of each valve is applied internally to the material of the balloon, the margin of which is caught between two rubber bands, and thereupon pressed against the seat by an external wooden hoop and bolts.
These valves do not present any external projection against which the cord of the net might catch.