Meteorology : comprising a description of the atmosphere and its phenomena, the laws of climate in general, and especially the climatic features peculiar to the region of the United States; with some remarks upon the climates of the ancient world, as based on fossil geology / by Samuel Forry.

  • Forry, Samuel, 1811-1844.
Date:
[1843]
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Licence: Public Domain Mark

Credit: Meteorology : comprising a description of the atmosphere and its phenomena, the laws of climate in general, and especially the climatic features peculiar to the region of the United States; with some remarks upon the climates of the ancient world, as based on fossil geology / by Samuel Forry. Source: Wellcome Collection.

Provider: This material has been provided by the National Library of Medicine (U.S.), through the Medical Heritage Library. The original may be consulted at the National Library of Medicine (U.S.)

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    now. It is, therefore, no ways suiprising that a moderate fall of snow- will sometimes entirely vanish during a succeeding northerly gale, without the slightest perceptible liquelaction on its surface. It has been already stated, as a general law, that the rate of evaporation, as well as the actual quantity of water in the state of vapor in the atmosphere, in- creases with the augmentation of temperature. The law of this in- crease, however, is not regular; but it will suffice the object now-in view, to state that, at all atmospheric temperatures below the boiling point of water, while the increase of temperature advances in arithme- tii i! progression, being s'ow and uniform, the corresponding rate of the elastic force of vapor, by which the quantity of water as vapor is deter- mined, increases so rapidly as to be nearly in geometrical progres- sion. Above 212°, the law of the elastic force of vapor is very dif- ferent from what it is below the boiling point. As water at 212° exists in the gaseous form, it obeys precisely the same laws and exerts the same elastic force, under similar circumstances, as atmospheric air does. But compared with atmospheric air at the temperature of 32°, how wide is the difference ! While at this temperature water is a solid, and the force of the elasticity of its vapor is equivalent only to about l-5th of an inch of mercury; the atmospheric air, on the other hand, may exert an elastic force equal to the weight of a column of mercury thirty inches high. If water is evaporated in closed vessels, it may be again obtained by the action of certain substances that have a great affinity for water; such as quicklime, chloride of lime, carbonate of potassa, and sulphuric acid. As they absorb the moisture of the air, the weight thus acquired has been taken as the measure of the quantity of its water. From some very careful experiments thus conducted by Saussure, he concluded that a cubic foot of air completely saturated with moisture, at the tempera- ture of 65° Fahr., holds eleven grains of water dissolved. But the method of Dr. Dalton, which consists in observing the temperature at which moisture begins to be separated from the air, ispreferable. This, which is termed the Dew-point, constitutes a valuable element in sev- eral important problems in meteorology. Subsection 2.—Condensation. % Intimately connected with evaporation are the phenomena of conden-1 sation of vapor from the atmosphere. As a reduction of its temperature causes a diminution of the capacity of the air for moisture, it follows that when vapor, at any given temperature, is cooled below the point of saturation, a portion is separated in the form of fluid water, while the j elastic condition proper to the newly acquired and diminished tempera-; ture7 is assumed by the remainder. Thus the temperature may be di-; minished by mingling with colder currents in the atmosphere, when a ; quantity of water is separated, which, according to the suddenness of j the change, will be either suspended in the form of visible clouds, or' precipitated in fogs, rain, or hail. These two processes of evaporation j and condensation, by abeautiful provision of nature, have aconstantten- dency to control each other's operations; for, while the former is in- creased by heat and produces cold, the latter is produced by cold and j liberates heat. Another arrangement no less wonderful is, the fact that! water raised by evaporation is freed entirely from all foreign substances, and is thus condensed in a state of absolute purity. It is thus seen that the degree and rate of evaporation, notwithstand- ing they increase with the temperature, are regulated chiefly by the existing degree of saturation of the air. Hence there can be neither | evaporation rtor condensation in an atmosphere perfectly saturated with moisture, and in a state of thermal and dynamical equilibrium. The processes of evaporation and condensation, says Prout. in his Bridge- water treatise, always indicate a disturbance of the thermal equilib-i rium in some part of the atmosphere : condensation denoting a depres-1 sion of the temperature below the mean, or point of thermal equilibrium i' evaporation, on the contrary, denoting that the temperature in some part of the atmospheie has been raised above the mean ; or at least that the temperature having been depressed below the mean, is again undergo- ing an elevation to the mean point. Evaporation and condensation may be thus considered as mutually dependent; so that one process cannot j lake place without the other. For this reason, in the great expanse of | nature, these two processes oscillate or fluctuate about the point of equi- librium, within certain limits which are neverpassed; and which limits, j though subject to countless anomalies, in general, decrease from the Equator toward the Poles. It thus appears from the preceding remarks that the only fluctuating] ingredient in the composition of the atmosphere as regards its permanent j gases, (for its oxygen and nitrogen have been found to be the same on j the summits of lofty mountains and in the narrow lanes of cities,) is its I aqueous vapor; and to determine this varying quantity is an object of | the highest importance. The principal property of this vapor, demand- j ing our attention, is its elasticity; and hence it becomes necessary to 1 bring under consideration the means employed to measure the elastic | force of atmospheric vapors. Subsection 3.—The Hygrometer. Hygrometers, or measures of moisture, are the instruments used for this purpose. They are of two classes, the one giving the measure immediately, and the other furnishing the result only indirectly and by means of inductions more or less uncertain. The former are gen- erally constructed on the principle of the condensation of vapor. The latter consist of a variety of animal and vegetable substances, which change in bulk from the absorption of moisture. For example, among: vegetables there have been employed for this purpose, a cord of flax | or hemp, which twists and untwists by moisture and dryness,—the beard of the wild oat, arena fatiut, which possesses a natural twist,— the Indian grass, andropogon contortion, cum midtis a/us. Among ani- m il substances, the most simple hygrometer is Wilson's, which consists of a rat's bladder, fastened to a glass tube, filled with mercury. By Saussure, a fiiie human hair, freed from unctious matter by boiling it in a weak alkaline solution, was employed ; and De Luc made use of a slip of whaleborle, scraped extremely thin. Both of these instruments were graduated by placing them in a jar filled with air saturated with moisture, and then in air dried by quicklime, or some other substance having similar properties; after which the space between these extreme points, was divided generally into one hundred degrees. Although indi- cating minute changes in the air, yet these substances are incapable of forcing comparable hygrometers ; to which must be added an equally fatal objection, that time alters their mobility and delicacy. Even the hair hygrometer of Saussure. notwithstanding both I lay Lussic andBiot gave to this instrument all the exactitude of which its construction is susceptible, is not entitled to confidence. Among the first class—those constructed on a simple and rigorous principle, giving at once the exact value of the element required—the hygrometer of Daniell takes 'he pre- ference decidedly. As it is quite portable and exact in its indications, it is a valuable meteorological instrument; but all of this class are liable to the objection that it is somewhat difficult, especially at low tempera- tures, to determine the precise degree at which dew begins to be depos- ited.* In Daniell's hygrometer, the only results which it is necessarv to enter on the register are, the temperature of the dew-point and tha't ef the surrounding air; for the first of these two data enables us, by refer- ence to tables calculated for this object, to determine the elastic force of the vapor and its weight; and the comparison of the two data gives the degree of humidity of the air at the time of observation. As air containing the same quantity of vapor does not necessarily possess the same degree of moisture, inasmuch as this quality depends upon the temperature of air at the time of observation, it becomes necessary to compare the temperature of the dew-point with that of the surrounding air. When the air is nearly saturated, a very slight diminution of tem- perature is attended with the formation of dew ; or, in other words, if these two temperatures are nearly equal, the air is almost saturated with vapor. But if, on the contrary, the air is dry, a body must be consider- ably colder before moisture is deposited on it; or, in other words, if the temperature of the surrounding air exceeds considerably that of the dew-point, the air is regarded as very dry. In a word, the less humid the atmosphere, the greater will be the difference between its tempera- ture and that of the dew-point. By attending to these two simple results, we may obtain all the elements required in the explanation of the mete- orological phenomena dependent on the presence of elastic vapor in the atmosphere. Another mode of obtaining a measure of the hygrometric state of the air is by comparing the indications of a thermometer with its bulb moist and dry—an idea which is due to Dr. James Hutton, the celebrated geologist. These are so constructed that two thermometers are mounted on the same scale, thus enabling us to see, at the same time, the indications of the wet and dry bulbs. The hygiometer, or double air-thermometer, of Sir John Leslie, is constructed on this principle; but it does not indicate the absolute dryness of the atmosphere, but merely the degree of dryness it has after being reduced to the temper- ature of the humid ball. We here determine the hygrometric condition of the atmosphere, by ascertaining the degree of refrigeration produced by the evaporation of water. An easy method of finding this, is to cover the bulb of the thermometer with a wet rag, and swing the instrument in the air for a few moments ; and the* noting the difference between this temperature and that marked by the dry thermometer. As regards the proper period of making hygrometrical observations, the author cannot recollect having ever met with very precise direc- tions, except in thope cases in which they are required to be made every one or two hours. But as few persons could be fouad disposed to engage in so tedious an undertaking, it is fortunate that by making two observations dailv, the three most important results may be ob- tained, viz., the maximum, the minimum, and the mean. Now as it is very probable that the maximum and minimam temperatures of the air in the 24 hours, correspond with the maximum and minimum tem- peratures of the dew-point, the proper times of observation are at three o'clock P. M., when the maximum temperature occurs, and in the morning between dawn and sunrise, when the lowest degree generally is found. The directions for observations on the wet bulb in the army of the United States, read thus: The hygrometric condition of the at- mosphere may be determined by ascertaining the degree of refrigeration produced by the evaporation of water. The most easy method of find- ing this, is to wet the bulb of a thermometer, covered round with fine gauze, and swing the instrument in the open air, in the shade, for a few moments, till the mercury sinks as low as it will. * * Water should not be poured upon the bulb of the thermometer, but appled with a bit of sponge, a fine brush, or any similar substance, and when the tem- perature is near, or below the freezing point, care should be taken simply to moisten the gauze. * * The wet bulb is taken at sunrise and at 3 P. M. Let us now turn our attention to the mode by which the elastic vapor of the atmosphere is reconverted into moisture. Subsection 4— Dew. This phenomenon may be thus explained, in a general way: When the direct influence of the sun is removed in the evening, the surface of the earth, in consequence of the ceaseless activity of caloric to maintain a state of equilibrium, radiates a portion of its superfluous temperature into surrounding space; and as the tem- perature of the air immediately in contact with the surface thus be- comes reduced below the point of saturation, a part of its water is condensed in the form of dew. Ever since the time of Aristotle, the phenomena and cause of this deposition have engaged the attention of philosophers; but until the comparatively recent experimental investigations of Dr. W. C. Wells, all our views on this subject were merely speculative. The fact that the bodies on which dew is deposited have invariably a lower tempe- rature than the ambient air, had been pointed out by Dr. Patrick Wil- son of Glasgow ; but while this coldness was supposed to be the effect of the deposition of dew, it was reserved for Dr. Wells to make the important discovery that it always precedes tlie formation of dew, and is in reality'the cause of this aqueous vapor. Prior to the appearance of Dr. Wells' elegant Essay on Dew, it was a disputed question among philosophers whether the phenomenon is produced by the rising of vapors from the earth, or by its descent from the atmosphere The circumstance that the glass-bells with which gardeners cover plants during the night, have, in the morning, their in- *Since writing the above, the author has satisfied hiiiself that Daniell's hygrometer is unadapted to the dry climate of the Unitrd States, with the exception perhaps of our southern borders. This conclusion was arrived at by a recent army medical board, after a full and patient investigation of th« subject.