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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    weather, when the rain-gauge cannot be used out of doors, it will be [ taken into the room, and a tin vessel will be substituted for receiving the snow, rain, or sleet, that may then (all. Tins vessel must have its. opening exactly equal to that of the rain-gauge, and widen downward to a sufficient depth with a considerable slope. It should be placed where nothing can obstruct the descending snow from entering, and where no dritt snow can be blown into it. During a continued snow- storm, the snow may be occasionally pressed down. The contents of the vessel must be melted, by placing it near the fire, with a cover to prevent evaporation, and the water produced poured into the gauge to ascertain its quantity, which must then be entered on the register. it'tion 9.—Snow. In regard to the formation of this substance, our knowledge is limited ; but in respect to the different forms o.L crystallization which the flakes assume, our observations are more complete. When careiully examined by a microscope, flakes of snow are found to consist of a mass of beau- tiful crystals, more or less perfect and regular These appearances were first described in 1740 br Dr. Nettia of Middleburgh ; but it is to Captain Scoresby, who availed himself of the opportunities for its investigation presented in his polar voyages, that we are chiefly indebted for our knowledge on this interesting subject. His work contains the delinea- tions of about 100 very curious and remarkaMe figures; but it were foreign to the object of this work to detail his results. Modifications or combinations of the hexangular prisms, often consisting of a star of six rays, formed of prisms united at angles of 60°, are exhibited by these flakes, the whole having a plumose appearance of exquisite beauty. The quantity of snow which falls in any place is regulated by its cli- mate, as depending on latitude, elevation, and position. In the polar regions, according to Mr. Scoresby, it snows nine days out of ten dur- ing April, May and June, the heaviest falls occurring when a moist cur- rent from the sea encounters a cold breeze from the surface of the ice. During the most inclement season, when the inhabitants of these inhos- pitable climes have immured themselves in their huts, it is necessary, he says, to prevent the admission of the cold air by stopping up every crevice, or otherwise the vapor of the confined air would be imme- diately precipitated as snow. As snow is a bad conductor of caloric, it shields vegetation and pre- serves the seeds intrusted to the earth, from the rigorous cold of the higher latitudes—a property well known to the husbandman from time immemorial; but it is »nly since the successful investigatioas of Dr. Wells in regard to dew, that science has been able to afiorrl a satisfac- tory explanation. It not only prevents the loss of terrestrial heat by radiation, but defends, the surface from the frigorific influence of very cold winds. That the air is warmer during a fall of snow, than before or after, is a well known fact, which is attributable to the extrication of heat in the sensible form during the transition of the vapor from a fluid to the solid state. Another striking instance of design on the part of Nature, is exhibited in the laws of atmospheric temperature as regards snow and ice ; for, were these laws the same as we observe in other bod- ies, every country liable to snow and ice would be annually destroyed by inundation. In the liquefaction of ice, as already shown, 140° of caloric become inappreciable by the thermometer; and henee? were not this immense quantity of caloric rendered latent in the liquefaction of snow, the greatest accumulation would at once be converted into water as soon as the atmosphere should rise above the temperature of 32°. There are also what are called frost-smoke, *pi<u'tir mow, and sleet. The first, which is not unfrequently witnessed in polar latitudes, consists of frozen particles ot water, which float in the atmosphere in the form of crystallized spiculae. The second, which is supposed to have its origin in causes similar to those which produce snow, is composed of minute needles, grouped and intermingled so as to have the appearance of a delicate bouquet. Sleet, which is the thin transparent layer of ice, fre- quently observed in winter covering the surface of the earth and other objects, is form'ed by rain which freezes as it falls upon the ground. Subsection 10.—Hail. As these phenomena, so formidable to the interests of the husbandman, is of very frequent occurrence in the south of France, the subject has been very closely studied by the philosophers of that nation. Hail seems to be the production of sudden and intense cold in the higher regions of the atmosphere; and hence a fall of hail is not un- common in warm climates, in which snow is utterly unknown. These frozen balls or hail-stones exhibit great variety as regards form and size. The ordinary size varies from l-10th to l-3rd of an inch in diameter; but occasionally they are much larger, being three or four inches in diameter, and weighing from ten to thirteen ounces. In June, 1811, an enormous hail-stone ft-11 during a storm, in England, which meas- ured 6£ inches in diameter, having the appearance of a congeries of fro- zen balls about the size of walnuts. As the icy nucleus doubtless has a temperature far below the freezing point, the hail must necessarily acquire magnitude as it descends, by condensing on its surlaee the vapor of the lower regions of the atmosphere. Hence they are always more or less rounded, and composed of concentric layers. Notwith- standing it usually hails but a few minutes, extending very rarely to a quarter of an hour, the quantity of ice which falls is so great as frequent- ly to eover the ground to the depth of several inches. A hail shower is often ushered in by a peculiar rattling noise, which has been compared to the sound produced by a number of bags of nuts suddenly emptied together on a floor. The clouds that bring hail have a remarkably ashy color, and are little elevated in the atmosphere ; and they are ot great extent, with rugged edges and enormous protuberances on the lower surface. Hail is sometimes accompanied with thunder; and it gene- rally precedes the rain of a storm, and is sometimes mingled with it. It is most frequently formed in the spring and summer, and during the warmest part of the day. In winter or during the night, it seldom hails. So great is the fury of hail-storms in France that whole districts are sometimes laid waste ; and these calamitous visitations are usually ac- companied by whirlwinds and the most terrific electrical phenomena. And here the hail-stones hive often an irreguUr shape, being angular masses of many pounds in weight, as though they were fragments of a thick sheet of ice. To guard against these destructive visitations, the ingenious device of the Parato.inerre has been used in France,—a con- trivance which consists in the erection of pointed electric conductors on the surrounding hills, with the view of abstracting the electricity silently and gradually from the air, and thus prevent those dangerous accumu- lations over the included districts. In hot climates, says Dr. Traill, in the Encyclopaedia Britannica, the descent of large masses of frozen water is still more common than in Europe, especially within the calms ofl'the western coast of Africa. The celebrated Volta accounts for the first formation of hail in the following manner:—The clouds are formed of hollow vesicles, the external surface of which is fluid. The myriads of these, which form the upper surface of a cloud, must undergo, toward the south, a strong evaporation, both on account of the intensity of the solar rays and the dryness of the air in which they swim. The elastic vapor thus produced by the solar heat must first saturate the dry air through which it passes, and at length, by the low temperature of some superior stratum, become again reduced into a vesicular state, forming another cloud, differing in its electrical condition from the first The upper cloud will have positive electricity, on account of that species of electricity being devel- oped during the precipitation of vapor, th« lower having changed its character to negative, in consequence of the evaporation it has under- gone. A diminished temperature at length may produce, between the clouds, icy particles, or hail in a nascent state, which the opposite elec- trical states of the upper and lower clouds will cause to oscillate, until, by gathering matter from the surrounding moisture, they become at length enveloped in compact and opaque ice, and attain a size which, overpowering the electric forces, compels them by gravity to descend. The most powerful argument against this theory is that urged by M. Arago, who asks how it is that the clouds, which may be supposed to attract each other as well as the hail, are not brought together. But no theory has so far embraced all the phenomena. A satisfactory explana- tion requires in the first place a knowledge whence the cold arises, by which the nucleus of each ball of hail is produced ; in the next place, a knowledge of the process by which these balls augment in size ; in the the third place, a knowledge of the force which sustains in the atmo- sphere masses of ice, weighing sometimes a pound or more, (but, as al- ready observed, they may form as they fall, and fall as they form ;) and, in the last place, a knowledge why electrical phenomena are so ener- getic, passing rapidly from one state to another, whenever the sky ex- hibits those low, ragged, swollen, and ash-colored clouds, which an- nounce the impending hail-storm. The theory of Mr. Espy in regard to the formation of hail and several other atmospheric phenomena, will be noticed in the sequel. ... All the preceding phenomena which belong to the precipitation of water from the atmosphere viz., Dew, Fogs, Clouds, Rain, Snow, and Hail, have a close relation with one another, depending no doubt on some general fundamental cause; and to arrive at a theory satisfactory in every point of view, further research is demanded. SECTION V.—Winds in General. In continuation of the accidental phenomena pertaining to meteor- ology, the extensive and complicated subject of winds will now be brought under examination. Whatever disturbs the equilibrium of the atmosphere produces wind ; and as the most important of these disturbing causes is the action of the sun, winds are produced chiefly by that law of fluid matter which com- pels the atmosphere to seek its level in obedience to the attraction of the earth. Atmospheric currents may be considered under two heads: the first being of a general character, extend over the whole globe; and the second being of a local nature, arise from a great variety of dis- turbing causes in physical geography, as influencing the equal distribu- tion of heat over the surface of the globe, or modifying the hygrometric and electric states of the atmosphere. The general currents of the atmosphere depend principally upon the unequal temperature of the poles and the equator, in connection with the earth's diurnal motion upon its axis. As the mean annual tempera- ture in the vicinity of the equator, at the level of the sea, is upward of 80°, while in the polar regions it is constantly below the freezing point of water, 32°, and as the entire pressure of the atmosphere, at the level of the sea, is the same over all the earth's surface_, being equivalent to that of a column of mercury about thirty inches in height, it follows, to maintain this equilibrium, that two grand currents will be in perpetual motion. As a given bulk of air at the level of the sea in the polar re- gions, must consequently be specifically heavier, than a similar volume at the level of the sea under the equator, it is obvious that the densa cool air of the polar and temperate regions naturally tends to take the place of the more rarefied air over inter-tropical regions, which, owing to its lightness, ascends and flows back again over the colder stratum, north and south toward eaeh pole, so as to preserve the equilibrium. Thus are established two currents, a polar and an equatorial, which, being as permanent'in their operation as the causes that produce them, constitute one primary element of the winds—phenomena of the utmost importance in the economy of nature, as tending to equalize the distri- bution of temperature over the surface of the globe. The other primary element of the winds is the result of the earth's diurnal motion upon its axis. If the earth were at rest, and its surface a plain, the northern hemisphere would, of course, experience a con- stant north wind, and the southern hemisphere a constant south wind; but as the earth is in an unceasing state of motion upon its axis from west to east, these currents are deflected from their northern and south- ern course, producing an easterly current in the atmospiiere. The ve. locity of every point on the earth's surface, in its diurnal rotation, in- creases from the poles to the equator, the ratio of which, according to Captain Basil Hall, is as follows: 0° - - - 1000 miles. 50° - - - 640 miles. 10 - - - 985 60 - - - 500 20 - - - 940 70 - - - 342 30 - - - 866 80 - - - 174 40 - - - 766 90 - - - 0 Thus while the velocity of any point on the equator is about 1000 miles an hour, the poles are quiescent; and from this extreme, the rate of motion gradually increases until it attains its maximum at the equa- tor. The under-current of air from the polar regions to the equator, is consequently influenced by a force acting at right angles to that which