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The envelope is a rigid one of aluminium, subdivided into forty-nine compartments, each of which contains a flexible ballonet.

Each ballonet is theoretically proof against leakage--in practice there is an almost inappreciable porosity, which hardly counts for a comparatively short period, say a month.

During the rest of the night the crew worked with a will--patching, mending, and "doping" the holed fabric and carefully testing each repaired ballonet with compressed air before refilling it with the precious brodium.

The buckling of the aluminium envelope resulted in ballonet after ballonet collapsing under the pressure of water.

This class rise with the forward ballonet empty, and inclined up by the bow.