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relative. That which is actually shewn by the thermometer and the best hygrometers, is merely the degree to which the substance contained in them. may be expanded by heat, or contracted by cold; but as the freezing and boiling of water, and certain states and changes of other substances, are found to take place or exist always at the same temperature, we can compare the tendency which one substance has to heat or cool another substance, with just as much correctness as we can tell differences of weight. In the hygrometer, although, from recent observations and researches, it appears to be the instrument most likely, in the end, to throw light upon the highlyinteresting subject of the weather, we have no fixed point either for absolute dryness, or absolute moisture.

In this instrument, indeed, there is one fixed point-that of saturation, or when the air will not receive any more moisture; and this is marked 0 in the scale of the hygrometer; but whether even in this state the atmosphere always contains the same quantity of water, in the same bulk, has not yet been established. By experiment, it is found that at forty degrees three-tenths of the thermometer, the actual quantity of water in 100 solid inches of air, is 178 thousandth parts, or somewhat less than two-tenths of a grain, and that, in these circumstances, the atmosphere contains 7.3 per cent. of the whole moisture which it is capable of holding in solution, at the temperature of 50 degrees. If, therefore, air containing this quantity of moisture, at the temperature of 50 degrees, were cooled down to 40 degrees and three-tenths, the hygrometer would sink to 0, and the evaporation from the surface of the moistened rag or paper would cease. Interesting, however, as are these experiments, and the deductions from them, they have not yet arrived at such degree of perfection as to enable us to establish any general rules or principles that can be useful in a popular form, neither have any means yet been found of determining when the atmosphere is entirely deprived of moisture; because, after a time, the quantity contained in it is so small, that there are no instruments delicate enough for measuring any alteration of it.

PLUVIMETER, or Rain Gauge.

The object of this instrument is to ascertain the quantity of rain that falls at any particular spot, and when it is well constructed, and placed in a proper situation, it does this with a great degree of accuracy. A very convenient one, perhaps the most convenient, may be constructed in this manner: Make a copper funnel, perfectly circular and smooth in the sides, and exactly five inches in diameter at its upper extremity; and let this be freely exposed to the weather with the upper surface quite level; and it is obvious that it will receive all the rain that falls upon a circle five inches in diameter. To the lower end of the funnel attach a copper tube, about thirty inches long and about 1 inches in diameter, with a stopcock at the lower extremity, opening into a glass tube of half an inch in

diameter, and having a scale of inches and tenths attached to it. It is obvious that any depth of rain falling in the funnel will fill a much greater portion of this glass tube; for the one being five inches in diameter, and the other only five-tenths of an inch, the funnel is ten times as wide as the tube. But it has this access both ways, and, therefore, the surface of the funnel must be ten times ten times, that is, 100 times the surface of the tube; and, consequently, if there fall even the thousandth part of an inch of rain in the funnel, it will fill a tenth of an inch in the tube; and thus the quantity of rain that falls, if it be but as much as shall wet the surface of the ground, will be determined with the greatest nicety. Still the instrument has its defects, and they are defects which it does not seem possible entirely to remedy. In the first place, there is no means of so constructing the funnel as to make it absolutely certain that the whole rain falls into it. If a small portion of the top be made straight up instead of sloping, it would prevent the escape of at least some part of that which, otherwise, would be dashed over the leeward side; but, if this be continued too far, the water would be exposed to a large surface in the funnel. In the second place, some part of the water will always stick to the funnel and to both tubes; and when the rain falls in a very small quantity and very gently, this may form so great a portion of it as to render the instru ment very inaccurate from, thirdly, the evaporation which takes place from the sides of the funnel, and tube, and surface of the water in the latter. This evaporation is the most serious inconvenience of all, because it very often happens that, while rain is falling, and falling in great quantity from the upper regions of the sky, the air at the surface of the ground, has a very strong evaporative power; so much so, that, in the course of a year, there has fallen about 524 inches of rain upon the top of a hill only 600 feet high, while but about 25 inches has fallen at the base of the same hill. Now though the attraction of the hill, which, floating as they do, freely in the atmosphere, must have attracted the clouds towards it, a certain portion of this difference may be explained, without any reference to absorption of the rain, in falling through the 600 feet of atmosphere; yet, a great deal of it must have been owing to that cause; and, therefore, there is every reason to conclude, that, because of this evaporation, the depth of rain, shewn by the gauge at the bottom of the hill, was much less than that which actually fell upon the surface of its funnel. What gives pretty strong confirmation to this opinion is, that the difference of the two gauges was always greatest in those years in which there was the most rain. When there fell 34 inches on the hill, there fell 20 on the plain. When there fell 46 on the hill there was 24 on the plain; and when there fell 31 on the hill there fell 28 on the plain, As these differences are by no means regular, it shows that, unless accompanied by careful observations of the hygrometer during the fall of rain, the indications of the rain-gauge are liable to many imperfections.

The ATMOMETER, or Evaporating Basin, is intended for showing the very opposite to the rain-gauge; for, as the gauge indicates the quantity of water that falls upon the ground, the basin is meant to show how much of that water is again taken up into the atmosphere by the process of evaporation. The simplest form of this instrument is a circular basin of the same width throughout, placed perfectly level, filled with water to such a height, that the wind may blow no part of it over the edge, and the quantity evaporated may be shown by a glass tube, communicating with the basin at the bottom, and having a scale attached to it. This instrument is, however, subject to many imperfections. When it is exposed to the sun it becomes heated, and thus the evaporation is greatly increased. It is also increased by exposure to the wind, which constantly brings a new stratum of the atmosphere over its surface; and the evaporating power of this new portion will be increased if it has previously passed over a dry surface, and diminished if it has passed over a moist one.

An atmometer has been contrived of a ball of porous earthenware, into the top of which there is inserted a glass tube, so divided, as that the sinking of the water one division in it, corresponds to the evaporation of the thousandth part of an inch from the surface of the ball. The upper end of the tube is closed so that the air cannot enter, and thus the water comes to the surface no faster than it is drawn there by the evaporative power of the air. This instrument is both ingenious and simple, but it has several imperfections, which cannot easily be rectified. In the first place, it cannot be used during frost; secondly, when rain falls it is absorbed by the ball; and, thirdly, as the ball gradually gets empty in dry weather, it cannot be used for any continued series of experiments.

The only other instruments which we shall notice, are the ELECTROMETER, for ascertaining the affections of the atmosphere as to electricity; and the ANEMOMETER, or wind-gauge, for measuring the force, and thereby finding the rapidity of the air when in motion.

Any substance which strongly attracts electricity, and is suspended by a connexion with the earth, which does not convey the electricity to that, may be called an electrometer; as, for instance, a paper kite with a piece of metal attached to it by a small wire which reaches nearly to the ground, the kite being held by a string of dry silk. In this situation the wire and metal attract electricity, and the silk string being what is called a nonconductor, does not convey the electricity so collected, to the ground. Upon applying any substance that will receive electricity to the metal, it is ascertained whether the atmosphere be in a more or less electric state than the substance so applied. If it be more electric, the excess will be communicated to the substance; and if it be less so, the deficiency will be taken from the substance. In neither case, however, are there any certain means of measuring the degree. We cannot tell what proportion the electricity collected by the wire and metal bears to the whole electricity in the

atmosphere; nor have we, indeed, any means of comparing two different states of electricity, so as to say the one of them is any number of times the other, in the case of excess, or any part of the other in the case of deficiency. All that we can do is, to say that the air is more or less electric than the earth; or that, in the case of the electrometer shewing no indication either way, that the electricity of the two is in a state of equilibrium.

Perhaps the best, and certainly the most philosophical instrument for measuring the velocity of the wind, is a common spirit-of-wine thermometer, having a large bulb. If the bulb be taken in the hand and heated, and then freely exposed to the wind, it is easy, from the time that it takes to cool, to deduce the rapidity with which the wind moves, upon the principle that the cooling power of the air must be in proportion to the rate of its motion, and there can be very little doubt of that, inasmuch as that is the measure of the new portions of air which are brought in contact with the bulb of the thermometer. Other anemometers are constructed by exposing a small windmill, the size of the sails of which is known, to the wind, and making it turn round an axle which tapers, and upon which a string with a weight to it is attached. As the string coils upon the thicker part of the axle, the weight is raised with greater difficulty, and from the point at which it overcomes the motion of the sails and stops them, the velocity of the wind may be calculated.

Such are the principal instruments for indicating changes in the atmosphere. With the exception of the barometer, which, when well constructed, shows the weight accurately; and the thermometer, which points out the temperature with considerable precision, they are all imperfect; although they are still in so far useful, in enabling us to form, at least, conjectures upon that interesting and every-day topic, the weather; and the knowledge of them, as well as that of the general principles of the atmosphere, as explained in the former part of this number, will serve as some sort of preparation for understanding how the atmospheric changes are brought about, and what are their effects upon the atmosphere itself, upon the earth, and, consequently, upon the feeling and comfort of mankind.

To comprise the whole doctrines of the atmosphere in one number would be impossible, consistent with any thing like even a correct outline. Hence, only the general properties, and the instruments for measuring their degree, have been explained in the preceding pages; for the other applications, and a further account of the uses of the atmosphere in the general economy of nature, the reader may consult the numbers of this series of the LIBRARY, on "The Weather" and "Climate."

THE

WIND AND WEATHER,

OR

THE CHANGES OF THE ATMOSPHERE, WITH THEIR CAUSES AND EFFECTS.

It requires no argument to prove that the changes of the weather are highly interesting: we feel them at every step, and they are the subject of our daily conversation; from them the poet derives some of the most sweet and also the most sublime of his images; they affect our spirits and our comfort, even when in health; and when sickness seizes or decay overtakes us, we are, as it were, at their mercy. When the heavens smile, the earth is invariably glad; and a gloomy sky fails not to sadden the whole face of nature. What can be more delightfully tranquillizing than to look upon the green earth and the blue sea, while the sky is without a cloud, the sun shining in all his glory, and there stirs not a wind to flutter the leaves or to ruffle the waters? What, again, can be more exhilarating than when the lively breeze sweeps along, when the light fleecy clouds sail along the sky, when the wind shakes perfume from every twig, and the waters dance to the music of nature? And, what can be more sublime than when the dark clouds roll their masses, when ruin seems to ride upon the wings of a thousand winds; when the lightning cleaves the sky and rends the rocks, when the voice of the thunder shakes the earth, and the sea is rolled into mountains, and its shores yeasted into foam? Even if we were mere spectators of these mighty changes, the contemplation of them would be delightful in itself, and would impress us strongly with the power and the wisdom of him who can bid the storm destroy, or hush it into repose-who can, in one moment, bid the lightning to strike, the thunder to roll, the wind to overturn, and the water to overwhelm, and the next moment command them all to be still. But, when each and all of these come home to our own feelings, when we are at their mercy upon the land, and, haply, our near relatives are at their mercy on the ocean, with but one thin plank between them and the unseen world, the study ceases to be abstract philosophy, and becomes a part, and an intimate and very important part, of our own history and economy.

Of the causes which produce changes of the weather, many are remote from our scrutiny; and we cannot ascend to

ARTS, No. 4.

"Rain's fountain-head, the magazine of hail,

the northern nests of feather'd snows, The brew of thunders, and the flaming forge, That forms the crooked lightning."

H

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