Page:My Airships.djvu/262

 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 (15 lbs. to the square inch), and it is equivalent to about 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), it 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 head of the balloon of