ball is about two-thirds filled with the water. The water having been introduced, the ball is set upon the fire, which gradually heats the contained water, and converts it into elastic steam, which rushes out by the pipe with great violence and noise; and thus continues till all the water is so discharged; though not with a constant and uniform blast, but by fits: and the stronger the fire is, the more elastic will the steam' be, and the force of the blast. Care should be taken that the ball be not set upon a violent fire with very little water in it, and that the small pipe be not stopped with any thing; for in such case, the included elastic steam will suddenly burst the ball with a very dangerous explosion.

EOLUS'S HARP, a very pleasing musical instrument, invented by Kircher. The construction is perfectly simple, consisting of little more than a number of catgut or wire strings, distended in parallel lines over a box of wood, with a thin top containing sound-holes. When the strings are tuned in unison, and the instrument is placed in a proper situation to receive a current of air, it produces, by the tremulous motion given by the wind to the strings, a soft, murmuring, and pleasing combination of sounds, which is beautifully described by Thomson in his Castle of Indolence. Fig.3.

ETIOLATION, a term denoting the state of vegetables which, by growing in the shade, and being deprived of light, become pale, white, and insipid. How this change is produced, the present state of our knowledge will not permit us to explain; but it is a fact of general observation, that the colour of herbs is pale or deep in proportion as they are less or more exposed to the rays of the sun; and those

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which, for the want of those rays, are pale or white, are said to be ætiolated, from a French word signifying star, as if they grew by star-light.

ERA, or Era, a fixed historical period whence years are reckoned: as the building of Rome, or the birth of Christ. See EPOCH.-Era and Epoch are not exactly synonymous. An era, is a point fixed by a particular people or nation; an epoch, one determined by chronologists and historians. The idea of an era, also, comprehends a certain succession of years, proceeding from a fixed event; and an epoch is that event itself. Thus, the christian era began at the epoch of the birth of Christ. See CHRONOLOGY.

AERONAUT, one who sails in the air. The term is applied to a person who ascends with an air-balloon.

AEROSTATION, aerial navigation, or the art of passing through the atmosphere in a buoyant state. Hence, also, the machines which are employed for this purpose are called aerostats, or aerostatic machines, and, from their globular shape, balloons. In the ancient history or romance of almost every nation, instances of persons travelling through the air have been related; and among the philosophers of Europe, since the revival of letters, the possibility of a mechanical contrivance, by means of which a man might rise into the air, or at least descend from heights with safety, has sometimes been discussed. The first experiments that may be strictly said to have introduced balloons, were made about the year 1766. In the year 1781, the Montgolfiers, considerable paper-manufacturers of Annonay, in France, raised a fine silk bag, of an elongated oval shape, to the height of seventy feet.

M. Pilatre-de-Rozier, who afterward fell a sacrifice to the project, was the first to ascend with a balloon. His voyage took place on the 15th of October, 1783, from the neighbourhood of Paris. The success of former experiments, induced M. M. Charles and Roberts to make an attempt, upon a principle somewhat different from those that had been employed, with a balloon of a spherical form. They rose from Paris, and descended at the distance of twenty-seven miles. There, M. Roberts left the boat or car; but the balloon still retaining a considerable quantity of inflammable gas, and its burthen being thus lightened, M. Charles resolved to take another voyage by himself. He was carried up with so much velocity, that in twenty minutes he was almost 9000 feet high, and entirely out of sight of terrestrial objects. At the moment of his parting from the ground, the globe had been rather flaccid; but it soon began to swell, and the inflammable air escaped from it in great quantity, through the silken tube. He frequently opened the valve, that it might be the more freely emitted, and the balloon effectually prevented from bursting. The heat of the inflammable gas being considerably greater than that of the external air, the former diffused itself all around, and was felt like a warm atmosphere; but, in the space of ten minutes, the thermometer indicated a variation of temperature as great as that between the warmth of spring and the ordinary cold of winter. M. Charles's fingers were benumbed by the cold; and he felt a violent pain in his right ear and jaw, which he ascribed to the dilatation of the air in these organs, as well as to the external cold. The beauty of the prospect which at this

juncture presented itself, made amends, however, for these inconveniences. At his departure, the sun was set on the valleys; but the height to which M. Charles was got in the atmosphere rendered its orb again visible, though only for a short time. He saw, for a few seconds, vapours arising from the valleys and rivers. The clouds seemed to ascend from the earth, and collect, one upon another, still preserving their usual form; though their colour was grey and monotonous for want of light in the atmosphere. By the radiance of the moon, he perceived that the machine was turning round with him in the air; and he observed that there were also contrary currents which brought him back again. He beheld, with surprize, from some unusual effect of the wind, the streamers of his banners pointed upward; a circumstance which, as he was moving horizontally at the time, cannot be attributed either to his ascent or descent. At last, recollecting his promise of returning to his friends in half an hour, he pulled the valve, to release a portion of the gas. When within 200 feet of the earth, he threw out 2 or 3 pounds of ballast, which rendered the balloon again stationary; but in a little time afterward he gently alighted in a field about 3 miles distant from the place where he set out; though, making allowance for all the turnings and windings of the voyage, he supposed that he had travelled 9 miles at least. By the calculations of Maunier, he rose 10,500 feet; a height somewhat greater than that of Mount Etna. During this voyage, the idea of guiding the machine by means of oars suggested itself to M. Charles; and this new experiment was made by M. Blanchard, who found, however, that

the strength he could apply in his apparatus was not great enough to counter-act, in any sufficient degree, the impression of the wind.

The only expedition in which a balloon has appeared to accomplish a practical purpose, was that of M. Blanchard and Dr. Jefferies, who, in the month of January, 1785, crossed the straits of Dover, and in the space of about 3 hours alighted safely in the forest of Guiennes. In the month of September, in the same year, Mr. Baldwin ascended from Chester in Mr. Lunardi's balloon. His account of the prospect which the earth afforded is extremely curious. At the height of what appeared 7 miles, though, by the barometer, it was only a mile and a half, he had a grand and most enchanting view of the city of Chester and its environs. The river appeared of a red colour; the city of a blue, and very diminutive. The whole looked a perfect plain; the highest building having no apparent height, and every thing seeming reduced to the same level. The lowest bed of vapour was of a pure white, in detached pieces, uniting as they rose: at the second height, the clouds appeared, to use Mr. Baldwin's expression, as a sea of cotton, tufted here and there by the action of the air. The prospect presently became an extended white floor of cloud, the upper surface being smooth and even. Above this white floor, Mr. Baldwin observed, at great and unequal distances, a vast assemblage of thunder-clouds, each parcel containing whole acres in extent, of the densest form; he compares their shape and appearance to the smoke of pieces of ordnance, consolidated, as it were, into masses of snow, and penetrating through the upper