"SANTOS-DUMONT No. 7"
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 outer balloon's rigidity remains inside the little interior balloon, but all the rest pushes its way out into the atmosphere again through its air valve, which opens at a little less pressure than do the gas valves.
Let us now return to the balloon of my "No. 6." The interior pressure on each square metre of its stem head being continuously about 30 kilogrammes the silk material composing it must be normally strong enough to stand it; nevertheless, it will be easy to see how it becomes more and more relieved of that interior pressure as the air-ship gets in motion and increases speed. Its striking against the atmosphere makes a counter pressure against the outside of the stem head. Up to 30 kilogrammes to the square metre, therefore, all increase in the air-ship's speed tends to reduce strain, so that the faster the air-ship goes the less will it be liable to burst out its head!
How fast may the balloon be carried on by motor and propeller before its head stem strikes the atmosphere hard enough to more than neutralise the interior pressure? This, too, is a matter of calculation; but, to spare the reader, I will content myself with pointing out that my flights over the Mediterranean proved that the balloon of my "No. 6" could safely stand a speed of 36 to 42 kilometres (22 to 27 miles) per hour without giving the slightest hint of strain. Had I wanted an air-ship of the proportions of the "No. 6" to go twice as fast under the same conditions its balloon must have been strong enough to stand four times its interior pressure of 3 centimetres of "water," because the resistance of the atmosphere grows not in proportion to the speed but in proportion to the square of the speed.
The balloon of my "No. 7" is not, of course, built in the precise proportions of that of my "No. 6," but I may mention that it has been tested to resist an interior pressure of much more than 12 centimetres of "water"; in fact, its gas valves open at that pressure only. This means just four times the interior pressure of my "No. 6." Comparing the two balloons in a general way, it is obvious, therefore, that with no risk from outside pressure, and with positive relief from interior pressure on its stem or head, the balloon of my "No. 7" may be driven twice as fast as my easy-going Mediterranean pace of 42 kilometres (25 miles) per hour, or 80 kilometres (50 miles).
This brings us to the unique and paradoxical weakness of the fast-going dirigible. Up to the point where the exterior shall equal the interior pressure we have seen how every increase of speed actually guarantees safety to the stem of the balloon. Unhappily, it does not remain true of the balloon's stern head. On it the interior pressure is also continuous, but speed cannot relieve it. On the contrary, the suction of the atmosphere behind the balloon, as it speeds on, increases also almost in the same proportion as the pressure caused by driving the balloon against the atmosphere. And this suction, instead of operating to neutralise the interior pressure on the balloon's stern head, increases the strain just that much, the pull being added to the push. Paradoxical as it may seem, therefore, the danger of the swift dirigible is to blow its tail out rather than its head in. (See Fig. 12.)
Fig. 12
How is this danger to be met? Obviously by strengthening the stern part of the balloon envelope. We have seen that when the speed of my "No. 7" shall be just great enough to completely neutralise the interior pressure on its stem head the strain on its stern head will be practically doubled. For this reason I have doubled the balloon material at this point.