In the year 1676, Cavendish investigated the properties of hydrogen gas, the levity of which made it exceedingly proba

Lana's Balloon.

Die that a thin substance filled with it would float in the air Cavallo made some experiments in 1782, but apparently gave up the subject in despair. In the same year Stephen and Joseph Montgolfier succeeded in constructing an apparatus which was lighter than the same bulk of atmospheric air, and consequently floated in it. After having made a number of experiments upon a small scale, they made public their discovery, and sent up a large balloon at Annonay on the 5th of June, 1783. It consisted of a large linen bag, lined with paper, 117 feet in circumference, and weighing 430 pounds. To the open end was attached a light wire basket, in which combustible substances were placed and inflamed, the heat so rarefying the enclosed air as to render it lighter than are ilk of the atmosphere. The machine ascended, and

at this time carried more than 400 pounds of ballast, rising in ten minutes to the height of 6,000 feet.

Interesting as these experiments were, but little was yet done that could be made serviceable for the purpose of aerial navigation, as future experiments proved. The fire balloon, or Montgolfier, as it is sometimes called, is very unfit to support the aerial voyager, however the car itself may be fixed. Pilatre de Rozier and the Marquis d'Arlandes did however make the perilous attempt, and ascending from the castle La Muette, rose to the height of 9,000 yards, and descended in safety. Several persons at different times repeated the experiment, but many have lost their lives in the indulgence of their curiosity or pride.

M. Charles, professor of natural philosophy in Paris, at last succeeded in making an air balloon. He provided himself with a bag of lustring, twelve feet in diameter, and coating it with a varnish of gum elastic, filled it with hydrogen gas. The apparatus weighed about twenty-five pounds, and, when set at liberty, rose to the height of 3,123 feet in two minutes.

The next improvement in the construction of balloons was Blanchard's inventon of an apparatus called the parachute, by which the aeronaut can, when required, regulate the velocity of his fall. M. Garnerin improved this apparatus, and thus greatly diminished the amount of danger arising from aerial excursions.

Among the aeronauts of our own country and time, Mr. Green is the most celebrated; but it may be doubted whether our capability of navigating the atmosphere will ever be found of any extensive service to man. The want of a method by which to control the direction of a balloon renders it at present worthless as a conveyance. The swiftness of its flight is a useless property to man, because it is the servant of the winds, and we have no power to stem the currents by which it is driven. Instances may occur in which it may be applied when the direction of the wind is favourable, but the opposing atmospheric currents, and their liability to change, give a doubtful character to every attempt. The result of Major Money's ascent proves the uncertainty of aerial advantages. This gentleman ascended from Norwich, the wind blowing at the time in such a direction as led him to suppose that he might fall in the neighbourhood of Ipswich. But he had scarcely attained the altitude of one mile before

he encountered a violent current, blowing in a new direction, which carried him towards Yarmouth.

The balloon fell in

The Parachute.

the sea about nine miles from land; the major supported himself for some time on the surface of the water by holding firmly upon the balloon, and he was at last relieved from his dangerous situation by a cutter that was cruising on the coast. This is not the only instance in which the aeronaut has been deceived by a change in the direction of the wind, and the existence of a superior current having a different course from that passing over the surface of the earth.

These general remarks on the causes of floating and sink. ing, as illustrated by the history of aeronautics, prove the fluidity of the air. But if a balloon had never been constructed, and man had never been poised between the earth and the ethereal vault, there would be no difficulty in proving the fact. Light bodies are conveyed from place to place upon

the aerial waves, the clouds float over them at all altitudes, and the feathered tribe rise and descend in the vast ocean,

as fishes do in the water, having physical arrangements of anatomy and constitution as beautifully adapted to the fluid in which their movements are to be performed, as those which characterize the finny tribes.

ELASTICITY OF AIR.

Elasticity is another property of atmospheric air, and in this statement it is assumed that it has compressibility, a property which it is well known to possess. Previous to Mr. Perkins's experiments, air had not been made to occupy a space less than the one hundred and twenty-eighth part of its ordinary volume on the surface of the earth; but this gentleman, by means of an apparatus invented for the purpose, succeeded in reducing it to a much less bulk, and states that he at last compelled it to assume a liquid state. The latter assertion, however, has generally been objected to, as his experiments are by no means satisfactory. It does not necessarily follow that a body possessed of compressibility should likewise be elastic, for some bodies, when compelled by pressure to occupy a less space than they do under ordinary cir

cumstances, retain that volume. But air is one of those bodies that have both these properties. If we take a syringe, closed at the end which is usually open, the piston may be driven down to considerable distance by the exertion of a little force. The air, therefore, in the tube or syringe suffers compression; but as soon as the pressure on the piston is removed, the air recovers its former volume, and the piston is forced back into its first position, from which fact we learn that air is possessed of the property of elasticity.

EXPANSIBILITY OF AIR.

Expansibility, or the capacity of occupying, under particular circumstances, a much larger space than it does under ordinary pressure, is another property of atmospheric air. Dr. Ure has calculated that the gases disengaged by firing gunpowder are so rarefied by heat that they occupy more than two thousand times the space of the powder itself; and Mr. Boyle caused atmospheric air to dilate until it had attained nearly fourteen thousand times its ordinary bulk.

There are two agencies which are especially active in expanding air, heat and a diminution of pressure. If a bladder containing only a small portion of air be exposed for a short time to the heat of a fire, or if boiling water be poured upon it, the air will expand, and the bladder appear as though it were fully distended by air in the ordinary state of density. So also, if it be placed under the receiver of an airpump, and a part of the air that presses upon it be abstracted, the remainder will expand and entirely fill the bladder.

From these statements it follows that air may have various densities, according to the circumstances under which it is placed. In the instance of condensation that has been mentioned, its density was great; in that of rarefaction, its density was small: and the same is true of the atmosphere; for its density at any height is just in proportion to the pressure that is exerted by the superincumbent mass of air. As the air is very elastic, it suffers, in the lower regions, where it bears a great pressure, considerable condensation, and extends itself as much in the higher regions, where there is no force to neutralize its elasticity. It therefore follows that the stratum of air immediately in contact with the surface of the earth is more dense than any above it, because it sustains a greater pressure, and its particles are consequently brought