Vapor

VAPOR. The late Prof. Henry, Secretary of the Smithsonian Institution, Washington, has contributed much valuable literature on meteor ology to what has been known before. The subject of vapor is an important one in agricul ture, and hence we extract, from an article by the distinguished scholar, what relates to vapor : The great motive power which gives rise to the various currents of the aerial covering of our globe is the unequal distribution of the heat of the sun; the elevated temperature of the equato rial regions beating the air causes it to ascend and flow over toward the pole, while the cold of the frigid zone produces a condensation of the air, which gives rise to downward currents in that region, and a spreading out there in all direc tions toward the equator. The simplicity of this movement is first interfered with by the motion of the earth upon its axis, which gives to all the currents flowing towards the equator a curvature to the west, and to all those flowing from the equator a curvature to the east. An other perturbing influence, which has been men tioned, is the unequal heating of the several parts of the different zones of the earth, consist ing as they do of alternations of land and water. But the great perturbing cause is the varying quantity of moisture which exists in the atmos phere, and which, by its increase and diminu tion, gives rise to the varying conditions of the weather, and produces the fitful and almost infinite variety of meteorological changes which occur at different times and in different places. We shall principally devote this article to an exposition of the phenomena of the vapor of the atmosphere, including that of the vaiious aque ous meteors, such as rain, hail, hurricanes, tor nadoes, etc. The meteorology of the United States, as well as its geology, is exhibited on a large scale, and affords one of the best fields on the surface of the globe for studying the gen eral movements of the atmosphere. The subject has attracted much attention on this side of the Atlantic, and a number of laborers have devoted themselves to it with ardor and success; but we regret that the discussions, which unavoidably arise among different investigators, have not always heen carried on with calmness and mod eration with' which the pursuit of truth should always be conducted. Indeed, meteorology has ever been an apple of contention, as if the violent commotions of the atmosphere induced a sym pathetic effect in the minds of those who have attempted to study them. We have no hypothe sis of our own to advocate; and while we attempt to reduce the multiplicity of facts which have been collected in regard to this subject to general principles, we shall aim at nothing but truth, and endeavor to select from the various hypothe ses which have been proposed, such as, in our judgment, are well founded on the estab lished laws of force and motion, and whieh give -the most faithful and explicit expression of the phenomena. We shall be ready at any time to modify or change our views as soon as facts are discovered with which they are incompatible, _and indeed we shall hold most of them as pro visional truths, which may serve to guide our inquiries, and which are to be established, modi fied, or rejected by the results of subsequent induction. The statement may be repeated, -vvhich has been previously made, that while the general principles of meteorology are well under stood, the facts relating to it, on account of the variations and multiplicity of condition, are the most complex of those of any branch of physical science. It has been properly said that astronomy is the most perfect of all branches of knowledge, because its elements are the most simple; and we may say, for a like reason, that meteorology is the least advanced, because its phenomena depend upon the concurrence of so many and such a variety of causes. The air at all times contains water in an elastic, invisible state, called vapor. To prove this, it is sufficient to pour into a bright metallic or glass tumbler a quantity of cold water, the outside of which will become covered with dew. If the vessel were pervious to the liquid, we might suppose the water which appears on the outside to come from -within, but this can not be the case vvith a metal lic or glass vessel, and the only source to which -vve can refer the dew is the atmosphere. The stratum of air immediately around the vessel is cooled by contact with its sides, and a portion of its vapor reduced to water. The air thus -cooled becomes heavier, sinks down along the side of the tumbler, and gives place to a new portion of which the vapor is also condensed; and in this way the process is continued as long as the temperature of the water is below that of the surrounding air. If the water which trickles down the side of the vessel is chemically exam ined, it will he found in some cases almost en tirely pure, and in others contaminated by animal and other effluvia which are diffused in the atmos phere. If the experiment be made on different

days and at different seasons, we shall find a _greater or less reduction of the temperature of the liquid within the tumbler is required in order to produce a deposition of the vapor. The greater the number of degrees of this reduction of tem perature, the greater will be the evaporation -from a given surface of water, and the more intense will be the different effects which depend on the relative dryness of the air. If the experi ment be made in summer, and we find but a small reduction of temperature is necessary to -produce the deposition of moisture on the outside ef the tumbler; and if we attend to the state of our feelings at the same time, we shall experi ence that peculiar sensation which is referred to -vvhat is called the closeness or sultriness of the atmosphere, and caused, by the large amount of vapor with which it is charged. To understand -even approximately the effects due to the vapor in the atmosphere, water in an aeriform condi tion as it exists by itself or separated from the atmosphere ; and for this purpose we may employ the ingenious method devised by Dr. Dalton, of Manchester, England, to whose researches in me teorology and other branches of physical science we are more indebted than to those of ahnost any ether individual of the present century. He employed in these researches, a glass tube of about forty inches in length, closed at one end, and filled with dry and warm mercury. The tube thus filled was inverted with its lower end in a basin of the same metal, and thus formed an arrangement similar to that of an ordinary barom eter, in which the pressure of the air, as is well known, forces up the mercury and keeps it sus pended at an elevation of thirty inches, when the experiment is made at the level of the sea. The space above the mercury is a Torricellian vacuum ; that is, a space void of all gross matter, save a very attenuated vapor of mercury, which can also be removed by a reduction of temperature below the fiftieth degree of Fahrenheit's scale, but the correction on this account is so small that it may be neglected. Into this vacuum Dr. Dalton introduced a very small quantity of water, by forcing it from a small syringe into the mercury at the, base of the column, whence it rose to the surface and was attended with an immediate depression of the mercurial column,which,when the temperature of the room was at 60°, amounted to nearly half an inch. By this experiment, it was proved that water, at the ordinary temper ature, when the pressure of the air is removed, immediately flashes into steam or vapor, and that the atoms of this vapor repel each other, thus producing an elastic force which depresses the column of mercury. In this experiment, the quantity of water introduced was but a few grains, yet it did not all flash into- vapor, but a portion of it remained in the form of a thin stratum of liquid on the surface of the mercury. Its weight, however, was insufficient to produce the observed descent of the column, and its effect in this respect could readily be calculated, since its weight was known. The descent of the mercury was therefore due to the repulsion of the atoms of vapor, and the former afforded an accurate measure of the comparative amount of this force. The tube, as we have stated, was forty inches long; and since the column of mer cury at first occupied but thirty inches of its length, the extent of the vacuum before the introduction of the water was ten inches, and afterward ten and a half inches. That the de pression of the mercury is an exact measure of the elastic force or repulsion of the atoms of the aqueous vapor, will be evident when we consider that if we remove the vapor the column will rise to thirty inches, and will then be exactly in equilibrio with the pressure of the external atmosphere; or, in other words, the two are in exact balance; but if, after the introduction of the vapor, the column is reduced half an inch in height, it is plain that the force which produces this effect must be just equal to the weight of this amount of mercury. Dr. Dalton next dimin ished the length of this vacuum by plunging the lower end of the tube deeper into the basin of mercury, and thereby causing the upper end of the column to be projected farther into the tube; but this produced no difference in the height of the column, the top of which was still depressed to half an inch below the normal height of thirty 'inches. From this experiment we infer that the repulsion of the atoms of vapor can not, like that of the atoms of air, be increased by external pressure; for when we attempt to coerce them into a smaller space by external pressure, a por tion of them is converted into water, and the atoms which remain in the aeriform condition exert the sarne amount of prestire as before. Dr.