Here's a little tidbit from Wonderful Balloon Ascents ©1870 about a hydrogen filled Hot Air Balloon.
The balloon is made of long strips of silk, sewn together, and rendered air-tight by means of a coating of caoutchouc. A valve is fitted to the top, and by means of it the aeronaut can descend to the earth at will, by allowing some quantity of the gas to escape. The car in which he sits is suspended to the balloon by a network, which covers the whole structure. Sacks of sand are carried in this car as ballast, so that, when descending, if the aeronaut sees that he is likely to be precipitated into the sea or into a lake, he throws over the sand, and his air-carriage, being thus lightened, mounts again and travels away to a more desirable resting-place. The idea of the valve, as well as that of the sand ballast, is due to the physician Charles. They enable the aeronaut to ascend or descend with facility. When he wishes to mount, he throws over his ballast; when he wants to come down, he lets the gas escape by the valve at the roof of the balloon. This valve is worked by means of a spring, having a long rope attached to it, which hangs down through the neck to the car, where the aeronaut sits.
The operation of inflating a balloon with pure hydrogen is represented in the engraving on the next page.
Shavings of iron and zinc, water, and sulphuric acid, occupy a number of casks, which communicate, by means of tubes, with a central cask, which is open at the bottom, and is plunged in a copper full of water. The gas is produced by the action of the water and the sulphuric acid upon the zinc and the iron: this is hydrogen mixed with sulphuric acid. In passing through the central copper, or vat, full of water, the gas throws off all impurities, and comes, unalloyed with any other matter, into the balloon by a long tube, leading from the central vats. In order to facilitate the entrance of the gas into the balloon two long poles are erected. These are furnished with pulleys, through which a rope, attached also to a ring at the top of the balloon, passes. By means of this contrivance the balloon can be at once lightly raised from the ground, and the gas tubes easily joined to it. When it is half full it is no longer necessary to suspend the balloon; on the contrary, it has to be secured, lest it should fly off. A number of men hold it back by ropes; but as the force of ascension is every moment increasing, the work of restraining the balloon is most difficult and exciting. At length, all preparations being complete, the car is suspended, the aeronaut takes his seat, the Words "Let go all!" are shouted, and away goes the silken globe into space.
The balloon is never entirely filled, for the atmospheric pressure diminishing as it ascends, allows the hydrogen gas to dilate, in virtue of its expansive force, and, unless there is space for this expansion, the balloon is sure to explode in the air.
An ordinary balloon, with a lifting power sufficient to carry up three persons, with necessary ballast and materiel, is about fifty feet high, thirty-five feet in diameter, and 2,250 cubic feet in capacity. Of such a balloon the accessories—the skin, the network, the car—would weigh about 335 lbs.
To find out the height at which he has arrived, the aeronaut consults his barometer. We know that it is the pressure of the air upon the cup of the barometer that raises the mercury in the tube. The heavier the air is, the higher is the barometer. At the level of the sea the column of mercury stands at 32 inches; at 3,250 feet—the air being at this elevation lighter—the mercury stands at 28 inches; at 6,500 feet above sea level it stands at 25 inches; at 10,000 feet it falls to 22 inches; at 20,000 feet to 15 inches. These, however, are merely the theoretic results, and are subject to some slight variation, according to locality, &c.