Returning to Paris he built for himself an “aërodrome,” provided with great sliding doors like the one at Monaco, and equipped with a hydrogen plant, constructive appliances, and everything needed for the rapid rebuilding or repair of air ships. It stood in a vacant lot surrounded by a high stone wall and was made of posts covered with red and white canvas, so that it looked like a great striped tent. Inside, the central stalls were 31 feet wide, 165 feet long, and 44½ feet high,[12] the whole enclosure having accommodation for seven dirigibles all inflated and ready for instant service. When completed, in the spring of 1903, it was at once used to harbor three new air ships. These were the _No. 7_, designed for racing contests; the _No. 9_, called the Runabout, a minim air ship used for calls and short pleasure trips; and the _No. 10_, called the Omnibus, intended for several persons, with ample supplies for a considerable journey.
The _No. 7_, which excelled its predecessors in length and bulk, was intended greatly to outstrip the best of them in velocity. The first air ship had attained fourteen miles per hour, the _No. 6_, nearly twenty miles an hour in winning the Deutsch prize, and over twenty miles per hour on subsequent occasions, though provided with a motor rated at only 12 horse power. The new vessel which had little greater resistance than _No. 6_, was to carry four times the internal pressure, or about 12 centimeters of water, and to be propelled by an engine of 60 horse power. The inventor expected therefore to attain a speed of between forty and fifty miles per hour. A very lofty expectation for that day, and one still unrealized for many years.
The racing air ship, or _No. 7_, was of cigar form, supporting a long car beneath, and generally resembling the _No. 6_, but slightly more tapering. Her length was six times her major diameter, and her volume 45,000 cubic feet. The envelope was made of two layers of the strongest French silk, four times varnished, and was built exceptionally thick at the stern, where the differential outward pressure is greatest in flight. The propulsion was effected by a 60 horse-power water-cooled four-cylinder Clément engine actuating two screw propellers 16½ feet in diameter, one in front the other at the rear of the car. The poise and maneuvering were to be controlled in the usual way, by means of the rudder and shifting weight. The inventor seemed not to realize that the bow of his vessel was too sharp to cleave the air with minimum resistance, though his predecessor, Jullien, in 1850, had discovered experimentally that a torpedo form is better for speed than the symmetrical spindle form used by Santos-Dumont in his racing vessel. He did, however, in time, learn that the torpedo form of hull is better for stability of forward motion, and hence adopted that form in his little _Runabout_.