If Dr. Ferguson had been able to make use of two balloons, his chances of success would have been increased, and if one happened to burst in the air, he could, by casting out ballast, save himself by means of the other. But the maneuvering of two balloons would have been very difficult when it was necessary to preserve an equal ascending power in both.
After much reflection, Ferguson, by an ingenious contrivance, united the utility of two balloons without their inconvenience; he constructed two of unequal size and enclosed one within the other. The exterior balloon, in which he adhered to the dimensions given above, contained a smaller one of the same shape, only forty-five and sixty-eight feet respectively, of horizontal and vertical diameter. The capacity of this interior balloon then was only 67,000 cubic feet. It floated in the fluid surrounding it. A valve opened from one balloon to the other, and admitted of communication between them.
This arrangement had this advantage viz., that if it were at any time necessary to let the gas escape, it could be let off from the larger balloon. Even if they were obliged to empty it altogether, the smaller one would remain intact; they could then detach the exterior covering a useless drag on them and the second balloon by itself would not offer the same resistance to the wind as a partially-filled balloon.
Furthermore, if by accident the outer balloon were injured, the other would be intact. Both balloons were made with striped taffetas from Lyons, coated with guttapercha. This resinous-gummy substance is perfectly waterproof, and is unaffected by acids or gas. The taffetas were placed side by side double, stretching upwards to the top, where almost all the weight was.
This envelope could retain the gas for an unlimited period. It weighed half a pound to nine square feet. Now as the surface of the exterior balloon was about. 11, 600 square feet, its envelope weighed 650 lbs. The envelope of the second balloon had a surface of 9,200 cubic feet, and weighed only 510 lbs.; altogether they weighed 1,160 lbs.
The netting to hold the car was made of the strongest hempen cord; the valves became objects of the most minute care, as if they had been the rudder of a ship. The car was
