its attraction does raise a small tidal wave in the atmosphere, which is indicated by the barometer, but its effect is scarcely perceptible. According to the calculations of Laplace, the joint action of the sun and moon is only capable of producing a tropical wind flowing westward at the rate of about four miles a day, and the effect produced by the conjoint actions of Jupiter and Venus, when nearest the earth, would be a very gentle breeze moving about a foot in fourteen or fifteen days, or about a mile in twenty years.

The invisible and perfectly elastic fluid which surrounds the earth is called the atmosphere, or atmospheric air. It appears to consist principally of two distinct expansible fluids, mechanically combined in different proportions, a single portion or atom of oxygen gas being united to three parts by weight, or four by bulk of nitrogen, with a very slight admixture of carbonic acid, perhaps one-thousandth part of the whole. Air was formerly considered as an elementary body, but the analysis of this rare medium is one of the finest discoveries of chemistry. The atmosphere, although apparently so rare and mobile, is nevertheless, capable of presenting great resistance to any obstacles to which it may be opposed. We shall see this more completely illustrated when we describe the phenomena attending hurricanes and other windy storms, but meanwhile it will answer our present purpose to simply refer to its use as a natural agent in propelling vessels by means of sails, and urging the sails of wind mills. Although it is, comparatively speaking, light, and in the ordinary acceptation without weight, yet we must not forget that it is now clearly demonstratable that the atmosphere which invests our earth, presses everywhere on its surface with a power of about 15 lbs. to the square inch. The famous Torricellian experiment proves this. It is well known that if the mouth is applied at one end of a small tube, the other end of which is immersed in water, that upon exhausting the air from the tube by the process called suction, the fluid rises swiftly and flows into the mouth; this is a philosophical experiment, but well known to every child. Now, if a person ignorant of the principle that caused the water to rise in the tube, should be asked for an explanation, he might answer

as Galileo did, that it was 66 Nature's abhorrence of a vacuum." It is however, effected by the pressure of the atmosphere. When an open tube is dipped at one end into the water, the liquid does not rise in the tube until the air within it is removed by exhaustion, as we have described, when immediately the fluid rises, because the atmosphere without the tube, pressing upon the mobile particles of the fluid, forces them up. Now it is evident that if the tube was long enough, and the exhaustion perfect, the pressure of the air upon the liquid outside would raise a column of water just so high that the weight of this elevated water would be equal to the pressure of the atmosphere upon the liquid at the bore or orifice of the tube. The column of water which can be thus raised or sustained, is generally about 33 feet high; and as such a column when its area is one square inch, weighs about 15 pounds, we infer that the atmosphere presses upon the earth with a force of about 15 pounds to the square inch. Mercury or quicksilver, being 14 times heavier than water, the atmosphere will support a column of this but about 29 or 30 inches in height; and when a tube a little more than 30 inches long is closed at one end, and filled with mercury, and then inverted into a basin also containing mercury, the column will remain suspended nearly thirty inches high. The mercurial column would continually remain at the same elevation, if the atmosphere was subject to no variations; but this is not the case; upon observation it is found subject to continual variation, almost always falling before a wind arises, and preceding rain, and again rising at the approach of calm and fair weather. The instrument thus becomes a baromeler, or measurer of of the weight of the air. It is an invaluable instrument on ship-board, giving indications of the coming tempest long before any change is detected in the appearance of the sky. Since the barometric column is wholly supported by the pressure of the atmosphere, communicated through the mobile particles of the fluid metal, to the open mouth of the tube, it obviously points out a method of determining heights. This however, is from several causes, a matter of some nicety; thus the density of the air decreases as we rise upward, owing to the extreme elasticity of air. It is well known that a piston may be thrust

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down a tube closed at one end, condensing the air before it, until it can absolutely be urged no farther, the included air resisting its descent as effectually as any metal; upon releasing the pressure it again expands to its original bulk. The stratum of atmosphere in immediate contact with the earth is subject to the entire pressure of the superincumbent mass, and is thus denser, i. e. has more atoms contained in the same space than the stratum immediately above, and the second stratum is denser than the third, and so on. This decrease of density gives a limit to the extent of the atmosphere.

The ancients imagined that our atmosphere reached at least as far as the moon, but the discovery of the weight and pressure of the air destroyed at once this magnificent vision. Comparing the length of the mercurial column with the density of the aerial medium, it follows that if the atmosphere is a uniform fluid, it cannot exceed the elevation of five miles. But the air being very dilatable, the higher portions sustaining as we have shown a diminished pressure, must swell upwards and occupy a proportionally greater space. This property removes the boundary of the atmosphere to a much greater elevation. A height of 42 miles would indicate a rarefaction of a thousand times, for it has been proved that the density decreases in a geometrical ratio, as the height increases in an arithmetical ratio, thus:

The famous Kepler first proposed a method of determining the height of the atmosphere by means of the twilight. After sunset there appears in the western sky a bright illumination called twilight, which is caused by the sun's rays shining upon the higher regions of the air. This twilight fades when the sun has sunk below the horizon a certain distance; thus, let C be the position of a spectator upon the earth, encompassed with its atmosphere, and S A B the lowest ray of the sun after sunset, B C being the horizon. The sun's rays would now illluminate the portion of atmosphere between A and B, producing twilight visible to a spectator at D, but not to one at C. Now the twilight, or this last ray

of twilight, S A B, expires whenever the arc DC, is equal to 9°,

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or one-fortieth of the whole circle. The angle BC O is evidently a right angle, or 90°, consequently, from the well known principle that the square of the longest side of any right angled triangle, as BO C, is equal to the sum of the squares of the other two sides; the square of B O, is equal to the square of O C, added to the square of B C. Now B O is the height of the atmosphere, added to the half diameter of the earth, and O C is also the semi-diameter of the earth, or 4000 miles nearly, whilst BC, which may be assumed equal to DC, is the one-fortieth of the circumference of the earth, or 600 miles nearly. Hence the square of B O is equal to 6002, plus 40002, which is 16,360,000, and the square root of this is 4045, which is the length of B O; subtracting DO, or 4000 miles, leaves 45 miles for B D, the height of the atmosphere. It is probable the atmosphere extends beyond this, as, unless the sky be overcast, there is total darkness in no climate, even at midnight, and therefore the atmosphere must extend to such a distance as to receive the most dilute glimmer after the sun has attained his greatest obliquity, and sunk 90° below the horizon; this would require an elevation of at least 1640 miles, and before the centrifugal force would balance the attraction of *gravitation, it is possible it might extend 22,000 miles, and yet, this is scarcely a twentieth part of the distance of the moon. If the atmosphere really spreads out, even to the first mentioned limit, it must, in its remote verge, attain a degree of tenuity which it would baffle the imagination to conceive.

It was soon conceived, after the discovery of the pressure of the air, that the height of the mercurial column would vary with

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the elevation at which it was observed. The experiment was tried by M. Perier, at the suggestion of his brother-in-law the celebrated Pascal, who himself living at Paris, was not so conveniently situated as M. Perier, who lived at Clermont, in Auvergne, in the immediate vicinity of several mountains, which modern geology proves to have once been active volcanoes. Early in the morning of the 19th of September 1648, (two hundred years ago) Perier with a few friends, assembled in the garden of a Monastery situated near the lowest part of the city of Clermont, where he had brought a quantity of mercury, and two glass tubes closely sealed at the top. He filled and watched them as usual, and found the mercury to stand in both tubes at the same height, namely 28 English inches; leaving one behind in the custody of the sub-prior, he proceeded with the other to the summit of the mountain, the Puy de Dome, and repeated the experiment. Here the party were delighted, perhaps we may say surprised, although they expected it, to see the mercury sink more than three inches under the former mark, and remain suspended at a height of only 24.7 inches. In his descent from the mountain he observed at two several stations, that the mercury successively rose, and returning to the monastery it stood at exactly the same point as at first, thus incontrovertably proving that it was the pressure of the atmosphere which balanced the suspended column.

We will close this chapter with a description of a barometer or baroscope, which any one who has access to a tin-shop can

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construct, it is an instrument of great delicacy in its indications. Let A B C D, be a vessel partly filled with water, in which the