THE WEIGHT OF AIR. 209 Gay-Lussac and Biot ascended in 1804; and Welsh four different times in 1852, guided by the veteran Green, who first made use of the coal-gas with which our streets are lighted to inflate his gigantic air-ships. In one of these ascents Welsh attained the height of 22,930 feet, than four miles, and nearly as high as the summit of the loftiest mountain on our globe. more As it is on a scientific expedition that we are going, we must take with us all requisite instruments, a barometer, thermometer, hygrometer, electrometer, and anything else we may suppose of service. We must first see that we rightly understand all our instruments. There is the barometer; it is intended to measure the weight of the air. "What! has the air any weight?" cries, perhaps, some astonished reader, and he becomes almost incredulous when gravely assured that it is pressing with a weight of fifteen pounds on every square inch of his body. A fish would find it difficult to believe that the element through which he is gliding has any weight. The diver, too, when reminded of the pressure of water, would answer, that the water, so far from feeling heavy upon him, actually buoys him up; and yet we all know for a certainty that water has weight, and exerts no small pressure on any vessel containing it. Thus it is with the atmospheric ocean. In both cases, the reason why a body immersed does not experience this pressure, is, that the fluid presses equally on all sides. That air has weight may be easily proved by exhausting a vessel by an air-pump, when it will weigh less than it did when full of air, every hundred cubic inches of air abstracted causing a diminution of about half a drachm (30-83 grs. at 60° F., and 29.92 bar. press.). Many other proofs will the air-pump give of the prodigious force of the atmosphere when not counterbalanced by air at a similar tension; bladders or glass vessels being broken to pieces by it, and the exhausted receiver fixed so that no one man could lift it. If we immerse the open end of a tube, perfectly empty of air, in any liquid, this liquid will be forced up the tube by the external pressure of the atmosphere. Water will thus be raised even till it form a column of thirty-three feet in height, before its weight counterbalances the pressure from below. Mercury, or, as it is frequently called, quicksilver, will, on account of its greater weight, rise in such a tube only to the height of about thirty inches; and were we to weigh an inch column of water of thirty feet in length, or of quicksilver of thirty inches, we should find both alike to be fifteen pounds. This, then, is the measure of the atmospheric pressure exerted upon a superficial inch. Our barometer is essentially the instrument just described exhausted glass tube closed at the upper end and dipping into mercury, the height to which the liquid metal rises, indicating the actual pressure of the atmosphere. Of the barometer, as a weather-glass, we hope to speak at another time. an The thermometer, as every one knows, is intended to measure heat or cold. Advantage is taken of the fact, that bodies, especially metals, expand when heated; and so by placing liquid quicksilver in a glass bulb, and making it as it expands ascend a thin tube of the same material properly graduated, the degree of temperature is easily read off. We take the hygrometer with us for the purpose of measuring the amount of moisture in the air. This instrument is of very various construction, but it generally depends on determining the dew-point, as it is termed, that is to say, the degree of cold which is requisite to cause moisture in the air to be deposited as dew. The amount of vapour of water which the air is capable of holding uncondensed increases in a certain proportion as the temperature rises, and this affords the data for the calculation. THE ELECTROMETER. 211 The electrometer may tell us something about the changes in electrical condition of the spaces through which we pass. It is essentially two pieces of gold-leaf secured side by side into a short metallic rod, which for the sake of safety is inserted in a wooden frame fitting the mouth of a glass bottle. Our protected gold leaves lying so amicably together under ordinary circumstances will retire from one another with mutual aversion directly they are brought near any object which is in a state of electrical disturbance. Now the cords which fix us to the earth are cut asunder, and we rise majestically. The earth receding from our view becomes a map full of animated objects beneath us. But we must not delay by looking at the fair and singular prospect; we have little time to spare from our meteorological observations. It is the weight and pressure of the atmosphere, of which we were just speaking, that are forcing us up; our imprisoned gas is lighter than the air around, and thus we rise, even as a cork would rise through the watery ocean. But our barometer is telling us that this pressure is diminishing; the air is becoming thinner or rarer. We learn the same, too, from the difficulty of breathing we are beginning to experience, like those who ascend high mountains; and from the expansion of the gas in the balloon which necessitates our opening the valve for the escape of some of it, lest the silken envelope should be rent. In about eleven thousand feet of perpendicular ascent the mercury has sunk to only half its previous height. We must not, however, rashly conclude that at double that altitude the air would be so thin as to exert no pressure; we are sure that there is a limit to the atmosphere, but the rarefaction of it proceeds at a decreasing ratio as we ascend, and there are reasons for believing that it extends to somewhere about forty-five miles from the earth's surface. The thermometer, too, is indicating a diminution in the temperature as we rise; so, indeed, are our feelings, making us wrap ourselves in our great-coats. Mr. Welsh tells us that "the temperature of the air decreases uniformly with the height above the earth's surface until at a certain elevation, varying on different days, the decrease is arrested, and for the space of from two thousand to three thousand feet the temperature remains nearly constant, or even increases by a small amount; the regular diminution being afterwards resumed and generally maintained at a rate slightly less rapid than in the lower part of the atmosphere, and commencing from a higher temperature than would have existed but for the interruption noticed." Rising above the region of clouds, we get into a space where the temperature is below the freezing point; and this explains to us why, even in the height of summer, lofty mountains are always covered with snow. If we have provided ourselves with winged animals, as Gay-Lussac did, we may now let them fly. He tells us, that at the height of 8600 feet a bee was liberated, and flew away humming briskly. A linnet set free at 11,000 feet, before adventuring into the rarefied medium, settled for awhile on the stays of the balloon, and then dashed downwards perpendicularly. A dove, after hesitating awhile, whirled in great circles, and swooped earthward through a mass of cloud. Floating majestically through the cold, clear air of these upper regions, with the sun shining in unbroken splendour across the deep blue heavens which have never known a cloud, we consider our various observations with the hygrometer, and also upon the mists through which we have passed; but these require much arrangement, and before we give the results of them we must descend to terra firma, and gain some breathing time. J. H. G. THE LYMINGTON BRINE SHRIMP,* AND THE FEZZAN WORM.† In the reservoirs belonging to the salt-works of Lymington, in Hampshire, there occurs in great abundance a very pretty little animal belonging to the entomostracous crustacea, somewhat resembling a minute shrimp. In 1755 Dr. Schlosser, a German naturalist, visited these salt-works, and upon examining the salterns was surprised to observe the whole cistern tinged of a red colour. The liquor which was contained in the cistern, was the brine previous to its being boiled, and contained nearly a quarter of a pound of salt to a pint of water. He salt, which is sufficient to kill almost every other living animal, these little creatures were sporting about in all the enjoyment of existence. They are very slender, linear in form, and do not exceed from a quarter to half an inch in length. Their head is furnished with two hornshaped appendages, and on each side of the body is a row of foliaceous-looking feet, eleven in number. "There is nothing," says Joly, another observer, "more elegant than Artemia salina, fig. 1. + Artemia Oudneyi, fig. 2. The figures marked a are the natural size. The other figures are magnified representations of the several parts. |