throughout warmer both in winter and summer, which therefore causes the Atlantic to flow into it; and the Black Sea being colder than the Mediterranean, flows into the latter. 2285. The eastern parts of North America, as appears from meteorological tables, have a much colder air than the opposite European coast, and fall short of the standard by about ten or twelve degrees. There are several causes which produce this considerable difference. The greatest elevation in North America is between the 40th and 50th degree of north latitude, and the 100th and 110th of longitude west from London; and there the most considerable rivers have their origin. The height alone is sufficient to make this tract colder than it would otherwise be; but there are other causes, and those are most extensive forests, and large swamps and morasses, each of which exclude heat from the earth, and consequently prevent it from ameliorating the rigor of winter. Many extensive lakes lie to the east, and Hudson's Bay more to the north; a chain of mountains extends on the south of the latter, and those equally prevent the accumulation of heat; besides, this bay is bounded on the east by the mountainous country of Labrador, and has many islands; from all which circumstances arise the lowness of the temperature, and the piercing cold of the north-west winds. The annual decrease of the forests for the purpose of clearing the ground, and the consumption for building and fuel, is supposed to have occasioned a considerable decrease of cold in the winter; and if this should be the result, much will yet be done towards bringing the temperature of the European and American continents to something like a level. 2286. Continents have a colder atmosphere than islands situated in the same degree of latitude; and countries lying to the windward of the superior classes of mountains, or forests, are warmer than those which are to the leeward. Earth always possessing a certain degree of moisture, has a greater capacity to receive and retain heat than sand or stones, the latter therefore are heated and cooled with more rapidity: it is from this circumstance that the intense heats of Africa and Arabia, and the cold of Terra del Fuego, are derived. The temperature of growing vegetables changes very gradually; but there is a considerable evaporation from them: if those exist in great numbers, and congregated, or in forests, their foliage preventing the rays of the sun from reaching the earth, it is perfectly natural that the immediate atmosphere must be greatly affected by the ascent of chilled vapors. 2287. Our next object is the ascent and descent of water: the principal appearances of this element are vapor, clouds, dew, rain, frost, hail, snow, and ice. 2288. Vapor is water rarefied by heat, in consequence of which becoming lighter than the atmosphere, it is raised considerably above the surface of the earth, and afterwards by a partial condensation forms clouds. It differs from exhalation, which is properly a dispersion of dry particles from a body. When water is heated to 212° it boils, and is rapidly converted into steam; and the same change takes place in much lower temperatures; but in that case the evaporation is slower, and the elasticity of the steam is smaller. As a very considerable proportion of the earth's surface is covered with water, and as this water is constantly evaporating and mixing with the atmosphere in the state of vapor, a precise determination of the rate of evaporation must be of very great importance in meterology. Evaporation is confined entirely to the surface of the water; hence it is, in all cases, proportional to the surface of the water exposed to the atmosphere. Much more vapor of course rises in maritime countries or those interspersed with lakes, than in inland countries. Much more vapor rises during hot weather than during cold: hence the quantity evaporated depends in some measure upon temperature. The quantity of vapor which rises from water, even when the temperature is the same, varies according to circumstances. It is least of all in calm weather, greater when a breeze blows, and greatest of all with a strong wind. From experiments, it appears, that the quantity of vapor raised annually at Manchester is equal to about 25 inches of rain. If to this we add five inches for the dew, with Dalton, it will make the annual evaporation 30 inches. Now, if we consider the situation of England, and the greater quantity of vapor raised from water, it will not surely be considered as too great an allowance, if we estimate the mean annual evaporation over the whole surface of the globe at 35 inches. 2289. A cloud is a mass of vapor, more or less opaque, formed and sustained at considerable height in the atmosphere, probably by the joint agencies of heat and electricity. The first successful attempt to arrange the diversified form of clouds, under a few general modifications, was made by Luke Howard, Esq. We shall give here a brief account of his ingenious classification. 2290. The simple modifications are thus named and defined: 1. Cirrus, parallel, flexuous, or diverging fibres, extensible in any or in all directions (fig. 248 a); 2. Cumulus, convex or conical heaps, increasing upwards from a horizontal base (b); 3. Stratus, a widely-extended, continuous, horizontal sheet, increasing from below (c). 2291. The intermediate modifications which require to be noticed are, 4. Cirro-cumulus, small, well-defined, roundish masses, in close horizontal arrangement (d); 5. Cirro-stratus, horizontal or slightly inclined masses, attenuated towards a part or the whole of their circumference, bent downward or undulated, separate or in groups consisting of small clouds having these characters (e). 2292. The compound modifications are, 6. Cumulo-stratus, or twain cloud; the cirrastratus, blended with the cumulus, and either appearing intermixed with the heaps of the latter, or superadding a wide-spread structure to its base (f); 7. Cumulo-cirro-stratus, vel Nimbus; the rain-cloud, a cloud or system of clouds from which rain is falling. It is a horizontal sheet, above which the cirrus spreads, while the cumulus enters it laterally and from beneath (g, g); 8. The Fall Cloud, resting apparently on the surface of the ground (h). 2233. The cirrus appears to have the least density, the greatest elevation, the greatest variety of extent and direction, and to appear earliest in serene weather, being indicated by a few threads pencilled on the sky. Before storms they appear lower and denser, and usually in the quarter opposite to that from which the storm arises. Steady high winds are also preceded and attended by cirrous streaks, running quite across the sky in the direction they blow in. 2294. The cumulus has the densest structure, is formed in the lower atmosphere, and moves along with the current next the earth. A small irregular spot first appears, and is, as it were, the nucleus on which they increase. The lower surface continues irregularly plane, while the upper rises into conical or hemispherical heaps; which may afterwards continue long nearly of the same bulk, or rapidly rise into mountains. They will begin, in fair weather, to form some hours after sunrise, arrive at their maximum in the hottest part of the afternoon, then go on diminishing, and totally disperse about sunset. Previous to rain the cumulus increases rapidly, appears lower in the atmosphere, and with its surface full of loose fleeces or protuberances. The formation of large cumuli to leeward in a strong wind, indicates the approach of a calm with rain. When they do not disappear or subside about sunset, but continue to rise, thunder is to be expected in the night. 2235. The stratus has a mean degree of density, and is the lowest of clouds, its inferior surface commonly resting on the earth in water. This is properly the cloud of night, appearing about sunset. It comprebends all those creeping mists which in calm weather ascend in spreading sheets (like an inundation of water) from the bottoms of valleys, and the surfaces of lakes and rivers. On the return of the sun, the level surface of this cloud begins to put on the appearance of cumulus, the whole at the same time separat. ing from the ground. The continuity is next destroyed, and the cloud ascends and evaporates, or passes off with the appearance of the nascent cumulus. This has long been experienced as a prognostic of fair weather. 2296. Transition of forms. The cirrus having continued for some time increasing or stationary, usually passes either to the cirro-cumulus or the cirro-stratus, at the same time descending to a lower station in the atmosphere. This modification forms a very beautiful sky, and is frequently in summer an attendant on warm and dry weather. The cirro-stratus, when seen in the distance, frequently gives the idea of shoals of fish. It precedes wind and rain; is seen in the intervals of storms; and sometimes alternates with the cirrocumulus in the same cloud, when the different evolutions form a curious spectacle. A judgment may be formed of the weather likely to ensue by observing which modification prevails at last. The solar and lunar halos, as well as the parhelion and paraselene (mock sun and mock moon), prognostics of foul wea ther, are occasioned by this cloud. The cumulo-stratus precedes, and the nimbus accompanies rain. 2297. Dew is the moisture insensibly deposited from the atmosphere on the surface of the earth. This moisture is precipitated by the cold of the body on which it appears, and will be more or less abundant, not in proportion to the coldness of that body, but in proportion to the existing state of the air in regard to moisture. It is commonly supposed that the formation of dew produces cold, but like every other precipitation of water from the atmosphere, it must eventually produce heat. 2298. Phenomena of dew. Aristotle justly remarked, that dew appears only on calm and clear nights. Dr. Wells shows, that very little is ever deposited in opposite circumstances; and that little only when the clouds are very high. It is never seen on nights both cloudy and windy; and if in the course of the night the weather, from being serene, should become dark and stormy, dew which has been deposited will disap. pear. In calm weather, if the sky be partially covered with clouds, more dew will appear than if it were entirely uncovered. Dew probably begins in the country to appear upon grass in places shaded from the sun, during clear and calm weather, soon after the heat of the atmosphere has declined, and continues to be deposited through the whole night, and for a little after sunrise. Its quantity will depend in some measure on the proportion of moisture in the atmosphere, and is consequently greater after rain than after a long tract of dry weather; and in Europe, with southerly and westerly winds, than with those which blow from the north and the east. The direction of the sea determines this relation of the winds to dew. For in Egypt, dew is scarcely ever observed except while the northerly or Etesian winds prevail. Hence also, dew is generally more abundant in spring and autumn, than in summer. And it is always very copious on those clear nights which are followed by misty mornings, which show the air to be loaded with moisture. And a clear morning, following a cloudy night, determines a plentiful deposition of the retained vapor. When warmth of atmosphere is compatible with clearness, as is the case in southern latitudes, though seldom in our country, the dew becomes much more copious, because the air then contains more moisture. Dew continues to form with increased copiousness as the night advances, from the increased refrigeration of the ground. 2299. Cause of dew. Dew, according to Aristotle, is a species of rain, formed in the lower atmosphere, in consequence of its moisture being condensed by the cold of the night into minute drops. Opinions of this kind, says Dr. Wells, are still entertained by many persons, among whom is the very ingenious Professor Leslie. (Relat. of Heat and Moisture, p. 37. and 132.) A fact, however, first taken notice of by Garstin, who published his Treatise on Dew in 1773, proves them to be erroneous; for he found, that bodies, a little elevated in the air often become moist with dew, while similar bodies, lying on the ground, remain dry, though necessarily, from their position, liable to be wetted, by whatever falls from the heavens, as the former. The above notion is perfectly refuted by the fact, that metallic surfaces exposed to the air in a horizontal position, remain dry, while every thing around them is covered with dew. After a long period of drought, when the air was very still and the sky serene, Dr. Wells exposed to the sky, 28 minutes before sunset, previously weighed parcels of wool and swandown, upon a smooth, unpainted, and perfectly dry fir table, 5 feet long, 3 broad, and nearly 3 in height, which had been placed, an hour before, in the sunshine, in a large level grassfield. The wool, 12 minutes after sunset, was found to be 14° colder than the air, and to have acquired no weight. The swandown, the quantity of which was much greater than that of the wool, was at the same time 13° colder than the air, and was also without any additional weight. In 20 minutes more the swandown was 144° colder than the neighboring air, and was still without any increase of its weight. At the same time the grass was 15° colder than the air four feet above the ground. Dr. Wells, by a copious induction of facts derived from observation and experiment, establishes the proposition, that bodies become colder than the neighboring air before they are dewed. The cold therefore, which Dr. Wilson and M. Six conjectured to be the effect of dew, now appears to be its cause. But what makes the terrestrial surface colder than the atmosphere? The radiation or projection of heat into free space. Now the researches of Professor Leslie and Count Rumford have demonstrated, that different bodies project heat with very different degrees of force. In the operation of this principle, therefore, conjoined with the power of a concave mirror of cloud, or any other awning, to reflect or throw down again those caloric emanations which would be dissipated in a clear sky, we shall find a solution of the most mysterious phenomena of dew. 2300. Rain. Luke Howard, who may be considered as our most accurate scientific meteorologist, is inclined to think, that rain is in almost every instance the result of the electrical action of clouds upon each other. 2301. Phenomena of rain. Rain never descends till the transparency of the air ceases, and the invisible vapors become vascular, when clouds form, and at length the drops fall: clouds, instead of forming gradually at once throughout all parts of the horizon, generate in a particular spot, and imperceptibly increase till the whole expanse is obscured. 2302. The cause of rain is thus accounted for by Dalton. If two masses of air of unequal temperatures, by the ordinary currents of the winds, are intermixed, when saturated with vapor, a precipitation ensues. If the masses are under saturation, then less precipitation takes place, or none at all, according to the degree. Also the warmer the air, the greater is the quantity of vapor precipitated in like circumstances. Hence the reason why rains are heavier in summer than in winter, and in warm countries than in cold. 2303. The quantity of rain, taken at an annual mean, is the greatest at the equator, and it lessens gradually to the poles; so there are fewer days of rain there, the number of which increase in proportion to the distance from it. From north latitude 12° to 43° the mean number of rainy days is 78; from 43° to 46° the mean number is 103; from 46° to 50°, 154; and from 51° to 60°, 161. Winter often produces a greater number of rainy days than summer, though the quantity of rain is more considerable in the latter than in the former season; at Petersburgh rain and snow falls on an average 84 days of the winter, and the quantity amounts to about five inches; on the contrary the summer produces eleven inches in about the same number of days. Mountainous districts are subject to great falls of rain; among the Andes particularly, it rains almost incessantly, while the flat country of Egypt is consumed by endless drought. Dalton estimates the quantity of rain falling in England at 31 inches. The mean annual quantity of rain for the whole globe is 34 inches. 2304. The cause why less rain falls in the first six months of the year than in the last six months is thus explained. The whole quantity of water in the atmosphere in January is usually about three inches, as appears from the dew point, which is then about 32°. Now the force of vapors of that temperature is 0·2 of an inch of mercury, which is equal to 2.8 or three inches of water. The dew point in July is usually about 58° or 59°, corresponding to 0.5 of an inch of mercury, which is equal to seven inches of water; the difference is four inches of water, which the atmosphere then contains more than in the former month. Hence, supposing the usual intermixture of currents of air in both the intervening periods to be the same, the rain ought to be four inches less in the former period of the year than the average, and four inches more in the latter period, making a difference of eight inches between the two periods, which nearly accords with the preceding observations. 2305. The mean monthly and annual quantities of rain at various places, deduced from the average for many years, by Dalton, is given in the following Table. September 4.874 4.350 1.617 1.842 1,550 4.140 3.135 October 1.780 4.741 3.537 2.573 2.118 2.460 2.502 2,816 2.286 2.512 3.697 3.663 3.006 4.140 3,665 3.311 2.435 4.581 3.281 3.654 2,289 3.751 3.922 3.724 3.079 4.151 November 3.360 3.441 2.634 3.775 December- 3.832 3.288 2,569 3.955 5.439 4.143 2.997 2.092 36.140 34.121 27.664 39.714 53.944 56.919 21.331 120.686 18.649 $3.977 2306. Frost, being derived from the atmosphere, naturally proceeds from the upper parts of bodies downwards, as the water and the earth; so the longer a frost is continued, the thicker the ice becomes upon the water in ponds, and the deeper into the earth the ground is frozen. In about 16 or 17 days' frost, Boyle found it had penetrated 14 inches into the ground. At Moscow, in a hard season, the frost will penetrate two feet deep into the ground; and Captain James found it penetrated 10 feet deep in Charlton island, and the water in the same island was frozen to the depth of six feet. Scheffer assures us, that in Sweden the frost pierces two cubits (a Swedish ell), into the earth, and turns what moisture is found there into a whitish substance, like ice; and standing water to three ells or more. The same author also mentions sudden cracks or rifts in the ice of the lakes of Sweden, nine or ten feet deep, and many leagues long; the rupture being made with a noise not less loud than if many guns were discharged together. By such means however the fishes are furnished with air, so that they are rarely found dead. 2017. The history of frosts furnishes very extraordinary facts. The trees are often scorched and burnt up, as with the most excessive heat, in consequence of the separation of water from the air, which is therefore very drying. In the great frost in 1683, the trunks of oak, ash, walnut, and other trees, were miserably split and cleft, so that they might be seen through, and the cracks often attended with dreadful noises like the explosion of fire-arms. 2308. Hail is generally defined as frozen rain, it differs from it in that the hailstones are not formed of single pieces of ice, but of many little spherules agglutinated together; neither are those spherules all of the same consistence; some of them being hard and solid, like perfect ice; others soft, and mostly like snow hardened by a severe frost. Hailstone has a kind of core of this soft matter; but more frequently the core is solid and hard, while the outside is formed of a softer matter. Hailstones assume various figures, being sometimes round, at other times pyramidal, crenated, angular, thin, and flat, and sometimes stellated with six radii, like the small crystals of snow. Natural historians furnish us with various accounts of surprising showers of hail in which the hailstones were of extraordinary magnitude. A a It differs from 2309. Snow is formed by the freezing of the vapors in the atmosphere. hail and hoar frost, in being as it were crystallised, which they are not. As the flakes fall down through the atmosphere, they are continually joined by more of these radiated spicula, and they increase in bulk like the drops of rain or hailstones. The lightness of snow, although it is firm ice, is owing to the excess of its surface in comparison to the matter contained under it: as gold itself may be extended in surface till it will ride upon the least breath of air. The whiteness of snow is owing to the small particles into which it is divided; for ice when pounded will become equally white. 2310. Snow is of great use to the vegetable kingdom. Were we to judge from appearance only, we might imagine, that so far from being useful to the earth, the cold humidity of snow would be detrimental to vegetation. But the experience of all ages asserts the contrary. Snow, particularly in those northern regions where the ground is covered with it for several months, fructifies the earth, by guarding the corn or other vegetables from the intenser cold of the air, and especially from the cold piercing winds. It has been a vulgar opinion, very generally received, that snow fertilises the land on which it falls more than rain, in consequence of the nitrous salts which it is supposed to acquire by freezing. But it appears from the experiments of Margraaf, in the year 1731, that the chemical difference between rain and snow-water, is exceedingly small; that the latter contains a somewhat less proportion of earth than the former; but neither of them contain either earth, or any kind of salt, in any quantity which can be sensibly efficacious in promoting vegetation. The peculiar agency of snow as a fertiliser, in preference to rain, may be ascribed to its furnishing a covering to the roots of vegetables, by which they are guarded from the influence of the atmospherical cold, and the internal heat of the earth is prevented from escaping. The internal parts of the earth are heated uniformly to the fifty-eighth degree of Fahrenheit's thermometer. This degree of heat is greater than that in which the watery juices of vegetables freeze, and it is propagated from the inward parts of the earth to the surface, on which the vegetables grow. The atmosphere, being variably heated by the action of the sun in different climates, and in the same climate at different seasons, communicates to the surface of the earth, and to some distance below it, the degree of heat and cold which prevails in itself. Different vegetables are able to preserve life under different degrees of cold, but all of them perish when the cold which reaches their roots is extreme. Providence has, therefore, in the coldest climates, provided a covering of snow for the roots of vegetables, by which they are protected from the influence of the atmospherical cold. The snow keeps in the internal heat of the earth, which surrounds the roots of vegetables, and defends them from the cold of the atmosphere. 2311. Ice is water in the solid state, during which the temperature remains constant, being 32 degrees of the scale of Fahrenheit. Ice is considerably lighter than water, namely, about one-eighth part; and this increase of dimensions is acquired with prodi gious force, sufficient to burst the strongest iron vessels, and even pieces of artillery. Congelation takes place much more suddenly than the opposite process of liquefaction; and of course, the same quantity of heat must be more rapidly extricated in freezing than it is absorbed in thawing; the heat thus extricated being disposed to fly off in all directions, and little of it being retained by the neighboring bodies, more heat is lost than is gained by the alternation: so that where ice has once been formed, its production is in this manner redoubled. 2312. The northern ice extends about 9° from the pole; the southern 18° or 20°; in some parts even 30°; and floating ice has occasionally been found in both hemispheres as far as 40° from the poles, and sometimes, as it has been said, even in latitude 41° or 42°. Between 54° and 60° south latitude, the snow lies on the ground, at the sea-side, throughout the summer. The line of perpetual congelation is three miles above the surface at the equator, where the mean heat is 84°; at Teneriffe, in latitude 28°, two miles; in the latitude of London, a little more than a mile; and in latitude 80° north, only 1250 feet. At the pole, according to the analogy deduced by Kirwan, from a comparison of various observations, the mean temperature should be 31°. In London the mean temperature is 50°; at Rome and at Montpellier, a little more than 60°; in the island of Madeira, 70°; and in Jamaica, 80°. 2313. Wind. Were it not for this agitation of the air, putrid effluvia arising from the habitations of man, and from vegetable substances, besides the exhalations from water, would soon render it unfit for respiration, and a general mortality would be the consequence. The prevailing winds of our own country, which were ascertained by order of the Royal Society of London, at London, are, The south wind blows more upon an average in each month of the year than any other, |