Page:Sim pall-mall-magazine 1904-01 32 129.pdf/27

 21 motive-power depends on two propellers each 4 metres in diameter and worked by a 60-horse-power motor with a water-cooler, has its envelope made of two layers of the strongest French silk, four times varnished, capable of standing, under dynamometric test, a traction of 3000 kilogrammes per linear metre.

I will now try to explain why the balloon-envelope must be made so very much stronger as the speed of the air-ship is designed to be increased ; and in so doing I shall have to reveal the unique and paradoxical danger that besets high-speed dirigibles, threatening them, not with beating their heads in against the outer atmosphere, but with blowing their tails out behind them !

Although the interior pressure in the balloons of my air-ships is very considerable, as balloons go—the spherical balloon, having a hole in its bottom, is under no such pressure — it is so little in comparison with the general pressure of the atmosphere, that we measure it, not by "atmospheres," but by centimetres or millimetres of water-pressure—i.e. the pressure that will send a column of water up that distance in a tube. One "atmosphere" means one kilogramme of pressure to the square centimetre ; and it is equivalent to 10 metres of water-pressure, or, more conveniently, 1000 centimetres of "water." Now, supposing the interior pressure in my slower "No. 6" to have been close up to 3 centimetres of water (it required that pressure to open its gas-valves), this would have been equivalent to $$\tfrac{1}{333}$$ of an atmosphere ; and as one atmosphere is equivalent to a pressure of 1000 grammes (1 kilogramme) on one square centimetre, the interior pressure of my "No. 6" would have been $$\tfrac{1}{333}$$ of 1000 grammes, or 3 grammes. Therefore on one square metre (10,000 square centimetres) of the stem, or head, of the balloon of my "No. 6,"the interior pressure would have been 10,000 multiplied by 3, or 30,000 grammes—i.e. 30 kilogrammes.

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 the 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 constantly