SPEED
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
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