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56 when not quite taut, containing 90 to 95 cubic feet of hydrogen. Hydrogen readily passes through paper, and the part of the experimental work that caused most trouble was the discovery of a suitable varnish, or "dope," to make the paper gas-tight. After many disappointments, a formula was arrived at, the application of which prevented appreciable evaporation of the gas for two or three hours, and which left a balloon with some lifting capacity after thirty-six hours.

The lifting power of a balloon is the difference between the weight of the hydrogen and the weight of the same bulk of air, less the weight of the balloon itself. The weight of the paper balloon was just over one pound; the available lifting power varied with the degree of tautness to which the balloon was filled, the height of the barometer and the temperature, but on the average, at ground level, the balloon as inflated would just support five and a half pounds. After a good deal of experiment the load of propaganda and releasing apparatus was fixed at four pounds and a few ounces, this allowing from 500 to 1,000 leaflets, according to their size, to be carried by each balloon, the balance of lifting power being sufficient to take the balloon sharply into the air to a