brass standard, and of that enclosed in the glass tube, with its pear-shaped bulb dipping into the ether, will prove that they both indicate the same temperature: this must be noted; and then from a small bottle, provided with a conical tube stopper, as shown in the next engraving, a few drops of ether are let fall upon the cambric envelop.

Ether being exceedingly volatile,-infinitely more so than water, will immediately gain heat from the bulb, for its conversion into vapor, or its evaporation; the bulb will be reduced in temperature, and the vapor of ether that it contains will be condensed therein into liquid ether, as was the vapor of the water, first condensed into liquid water in the experiment with the "Cryophorus."

As in that case, a call or demand is now immediately made upon the bulb containing the thermometer and the liquid ether, for a portion of vapor to supply the place of that which has been condensed; and there being no air in the instrument to oppose this demand, it is instantly supplied by gaining heat from the bulb in which the ether is contained.

t

This vapor in its turn is instantly condensed; but as the ether evaporates from the capillary attraction of the cambric, another, and another, and many calls, are so quickly made, that the bulb containing the ether, by

yielding heat, becomes extremely cold, as its included thermometer will indicate,—and at length becomes so cold, that the watery vapor contained in the atmosphere coming into contact with it, is condensed, as a fine ring of dew upon its surface at the level of the remaining ether.

This is instantly detected, by the strong relief into which the transparent bulb is thrown by the dark object, and the included thermometer shows the degree of temperature at which this phenomenon ensues; it must be immediately noted, and it is technically called "The dew-point."

1

Suppose the external thermometer indicate 60 degrees, and the internal thermometer indicate 48 degrees, the difference between the two is 12 degrees, and therefore the atmosphere does not contain a very large amount of watery vapor, since a reduction of temperature so considerable, is required for the deposition of dew; but suppose the one thermometer indicate 66 degrees, and the other 64 degrees, the difference between the two is only 2 degrees, and therefore the atmosphere does contain a very considerable amount of watery vapor, since it is immediately prone to deposit dew by such a slight reduction of temperature; therefore, in the first example, the degree of its dryness may be denoted by 12, and in the second by 2 degrees.

Or, the fact may be stated in the following terms, namely the more widely that the thermometers differ in their indications, the smaller is the quantity of the vapor in the atmosphere; and the more closely that they agree, the greater is the quantity of the vapor contained in the atmosphere.

For example; in exceedingly dry weather, the cam

178

brass standard, e with its pear-sl. prove that th must be not

with a conica graving, a f bric envel

ated with ether more e sufficient cold to sum· <spheric vapor to chill and other hand, in exceedingly a very few drops of ether y summon the large quantity mosphere is almost saturated, ,--to condense and deposit as

these facts may be gained, by ygrometer" in a hot dry room, will be observed between the noment of the "dew point;" and

cellar, when little difference will hem at the moment of the "dew he air of the cellar be already na...h vapor, or the air of a small apartsaturated with vapor, by a large wet ang within it, the two thermometers e at the moment of the deposition of , the "dew point."

a degree of atmospheric dryness, or ery vapor, as measured by the therhygrometer," seldom amounts to 30 、o say, the “dew point" is seldom 30 de temperature of the air; but in the ,,—an island which is situated near

of the river Ganges,-at a temperature e" dew point" has been seen as low as A marking the dryness or aridness of that mosphere at 61 degrees.

wientific investigations, accurate experiA made with the "hygrometer," and

elaborate tables have been constructed, to show the actual weight of aqueous vapor locally contained in the attosphere, at the time that the “dew point" is ascertained: but these tables or calculations, although highly interesting and important to professed meteorologists and chemists, would scarcely be intelligible to general readers and juvenile students; therefore no reference is made to them during this slight inquiry regarding the phenomena of the Four Seasons.

What has been stated concerning the construction and method of employing the “hygrometer,” the manner in which it may be caused to act as "a weather-glass,”— in the popular acceptation of the term,-to indicate the dryness and dampness of the atmosphere, and therefore the lesser or greater probability of rain, may be sufficiently intelligible.

Watery vapor, although invariably present in the whole atmosphere, and its mean quantity expressed by the enormous sum at page 28, is liable to local variations of its quantity, according to the local temperature of any particular volume of the atmosphere; for instance, it is shown at page 130, that warm air, either calm, or moving as a breeze, or as a high wind, is capable of evaporating and sustaining in its invisible volume more water than cold air, under similar conditions of repose and motion.

It has been ascertained, during most refined, accurate, and laborious experiments, conducted both by the meteorologist and the chemist, that a cubic foot of absolutely dry or anhydrous air, i. e., air consisting of Nitrogen, Oxygen, and Carbonic acid, when at the temperature of 33 degrees, or near the “freezing point" of water, does not absorb or sustain more than two grains and one-third of a grain of watery vapor; whilst at the temperature of

60 degrees, by such increase of heat, it will sustain five grains and three quarters of a grain; and at the temperature of 70 degrees, by such further increase of heat, it will sustain eight grains.

This experimental fact immediately accounts for the greater rapidity with which a pool, or other accumulation of stagnant water, is "dried away"—even by the naturally hydrated atmosphere-in warm weather than in cold weather; and it also, in a great measure, accounts for the more copious deposition of dew from the atmosphere in the one season than in the other.

In some cases, a parched volume of the atmosphere in motion, as "wind" traveling from one clime to another, over the wide expanse of sea or ocean, incites the rise, and accumulates the burden of so large a proportion of pure watery vapor upon its "wings," as to be at length unable to sustain the whole; and then, during its flight or transit, a portion is slowly and gently relinquished in the mysterious form of clouds.

These eventually descend as rain upon the earth; but in other cases, a parched volume or blast of wind is ordained to sweep over arid sands or deserts, and in place of refreshing these with a copious supply of water, it actually deprives them of their very scanty store of this invaluable fluid for its own hydration, and thus often causes a most intense and almost insupportable sensation of heat.

These phenomena are most particularly evident in Eastern climates, and being familiar to the most ignorant observers, they were chosen by the Savior of mankind as a medium for conveying a just and powerful reproof to the captious malevolence of benighted minds, concerning the utter vanity of physical observation and