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 420 cubic metres (nearly 15,000 cubic feet). In form it was a compromise between the slender cylinders of my first constructions and the clumsy compactness of the "No. 3." (See Fig. 7.) For this reason I thought it prudent to give it an interior compensating air balloon fed by a rotary ventilator like that of the "No. 2," and as the balloon was smaller than its predecessor I was obliged to return again to hydrogen to get sufficient lifting power. For that matter, there was no longer any reason why I should not employ hydrogen. I now had my own hydrogen gas generator, and my "No. 4," safely housed in the aerodrome, might be kept inflated during weeks. In the "Santos-Dumont, No. 4," I also tried the experiment of placing the propeller at the stem instead of the stern of the air-ship. So, attached to the pole keel in front, the screw pulled,