Page:My Airships.djvu/265

 my "No. 6" the interior pressure would have been 10,000 multiplied by 3, or 30,000 grammes i.e.—30 kilogrammes (66 lbs.). How is this interior pressure maintained without being exceeded? Were the great exterior balloon filled with hydrogen and then sealed up with wax at each of its valves, the sun's heat might expand the hydrogen, make it exceed this pressure, and burst the balloon; or should the sealed balloon rise high, the decreasing pressure of the outer atmosphere might let its hydrogen expand, with the same result. The gas valves of the great balloon, therefore, must not be sealed; and, furthermore, they must always be very carefully made, so that they will open of their own accord at the required and calculated pressure. This pressure (of 3 centimetres in the " No. 6 "), it ought to be noted, is attained by the heating of the sun or by a rise in altitude only when the balloon is completely filled with gas: what may be called its working pressure — about one-fifth lower—is maintained by the rotary air pump. Worked continually by the motor, it pumps air continually into the smaller interior balloon. As much of this air as is needed to preserve the