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 interior balloon, it would relieve the tension of the swelling hydrogen; and only afterwards, should this not be sufficient, would the stronger gas valves (Fig. 10) let out precious hydrogen. All three valves were automatic, opening outward on a given pressure from within. One of the hypotheses to account for the terrible accident to the unhappy Severo's dirigible "Pax" is concerned with this all-important problem of valves. The "Pax," as originally constructed, had two. M. Severo, who was not a practical aeronaut, stopped up one of them with wax before starting on his first and last voyage. In view of the decreasing pressure of the atmosphere as one goes higher the ascent of a dirigible should always be slow and never great, for gas will expand on the rise of a few yards. It is quite different from the case of the spherical balloon, which has no interior pressure to withstand. A dirigible whose