Mr. Green, who has made more than two hundred successful ascents, first employed the ordinary coal gas, in lieu of hydrogen, or heated air, to gain the buoyant power. As the specific gravity of this is considerably greater than that of hydrogen, it is necessary to employ a large balloon when coal gas is used. Mr. Green's present balloon contains about 24,000 cubic feet. There is much economy in using this substitute, as the cost price of sufficient coal gas does not exceed twenty pounds; but the expense of generating as much hydrogen gas as the balloon requires, amounts to nearly a hundred pounds. The latter product of the distillation of coal, when the more valuable illuminating gas is evolved, answers the purpose best, as it contains much less carbon, and is, con sequently, considerably lighter. The best material for constructing a balloon, is the silk stuff called lustring. When cut into gores it should be stretched by weights, or other means, and kept in this state several hours before it is var nished or sewn. It is usual to apply boiled linseed oil, containing a small quantity of oxide of lead, for the first coat of varnish, and a solution of caout chouc (Indian rubber) in oil of turpentine, for the last. The seams should be doubly sewn and overlapped ; and, when finished, a hot smoothing-iron should be passed over them, the silk being protected by an intervening sheet of paper. The gores should be cut with great precision. A net-work of strong hempen twine should be accurately fitted to the balloon, the meshes at the upper extremity being smaller than those below. To the separate cords which terminate the net-work, a circular hoop is attached, from which the car is sus pended by strong ropes. The whole of the net-work and ropes should be so adjusted, that the strain, or pressure, is equalized over the whole surface of the balloon. The valve for permitting the escape of gas must be retained in its place by a sufficient spring; and to render it perfectly air-tight, a luting of wax and oil is generally employed around the edges. The cord by which the valve
is opened passes through the interior of the balloon ; and great care is neces sary to prevent it being entangled. The extremity of this cord should never be fastened to the car, as in the event of its oscillation, the valve might be opened, and retained in that state until the greater part of the gas escaped. A grap pling-iron, attached to a strong rope, is an indispensable appendage to a bal loon. As much ballast as can be taken should always be employed. It frequently occurs that the aerial voyager is far above the clouds, and loses sight entirely of the earth by their intervention ; on passing through them, he may find that the spot on which he is descending is unfavourable. A few pounds of ballast thrown out, will occasion the balloon again to ascend, and pass over the inappropriate landing-place. The writer of this article was thus situated ; and all the ballast being discharged, he was in imminent danger of descending in an arm of the sea. A contrary current, near the surface of the earth, however, gave the balloon another direction, when escape seemed almost impossible. The appearance of the scenery below to an aeronaut is inexpressibly beautiful ; but as a bird's-eye view only can be taken, no correct opinion can be formed of the altitude of buildings, hills, or trees. On this account, much of the grandeur of an extensive prospect is lost. The chief inconvenience experienced is from the sudden variations of temperature. The upper regions of the atmosphere are intensely cold when the sky is cloudless, even although the sun be shining in meridian splendour. In an ascent made in August, when the thermometer was 740 in the sun at the time of ascent, the writer observed, in less than twenty minutes, that the mercury indicated a temperature of 310.