electricity. The first successful attempt to arrange the diversified forms of clouds, under a few general modifications, was made by Luke Howard, Esq. brief account of his ingenious classification. We shall give here a 2357. The simple modifications are thus named and defined: 1. Cirrus, parallel, flexuous, or diverging fibres, extensible in any or in all directions (fig. 207. 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). 2358. The intermediate modifications which require to be noticed are, 4. Cirro-cumulus, small, well defined, roundish masses, in close horizontal arrangement (d); 5. Cirrostratus, 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). 2359. The compound modifications are, 6. Cumulo-stratus, or twain cloud; the cirrostratus 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, or 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). 2560. 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. 2561. 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. Previously 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 ap. proach 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. 22 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 comprehends 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 separating 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. 2363 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 en 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 cirro-cumulus 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 haloes, as well as the parhelion and paraselene (mock sun and mock moon), prognostics of foul weather, are occasioned by this cloud. The cumulo-stratus precedes, and the nimbus accompanies rain. 2364. 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. 2365. 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 disappear. 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. 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 vapour. 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. 206. 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 alittle elevated in the air often become moist with dew, while similar bodies, lying on the ground, remain éry, though necessarily, from their position, as 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 130 colder than the air, and was also without any additional weight. In 20 minutes more the swandown was 144° colder than the neighbouring 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 neighbouring 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. 2367. 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. 2368. Phenomena of rain. Rain never descends till the transparency of the air ceases, and the invisible vapours become vesicular, 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. 2369. The cause of rain is thus accounted for by Hutton and Dalton. Also If two masses of air of unequal temperatures are, when saturated with vapour, intermixed by the ordinary currents of the winds, a precipitation ensues. If the masses are under saturation, then less precipitation takes place, or none at all, according to the degree. the warmer the air, the greater is the quantity of vapour precipitated in like circumstances. Hence the reason why rains are heavier in summer than in winter, and in warm countries than in cold. 2370. The quantity of rain, taken at an annual mean, is the greatest at the equator, and it lessens gradually to the poles; at which there are fewer days of rain, the number increasing in proportion to the distance from them. 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°, 134; 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 fall 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. 2371. 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 vapours 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. Thus it is evident that, in the latter month, the atmosphere contains four inches of water 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. 2372. 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; March April May June July August Inch. Inch. Inch. Inch. Inch. Inch. Inch. Inch. January 2:310 2.177 2.196 S:461 5-209 $-095 1:595 1:464 1-228 2:477 February 2:568 1.847 1-652 2.995 5.126 2.837 1741 1.250 1-232 1:700 2:098 1:523 1522 1.753 3:151 2164 1184 1.172 1:90 1-927 1748 2010 2:104 2:078 2.180 2.986 2:017 0:979 1-279 1-185 2656 1950 2.895 2573 2118 2:460 3:480 2:568 1-641 1636 19767 2931 2:407 2:502 2.816 2-286 2:512 0792 2.974 1843 1.758 1697 2-562 2-$15 3.697 3.663 3:006 4:140 4-959 3.256 2.903 2:448 1:800 1882 S-115 3.665 3.311 2435 4:581 5-089 3.199 2746 1-807 1.900 2:547 8.103 September 3.281 3.654 2-289 3751 4:874 4:350 1617 1.842 1:550 4:140 3-185 October 3922 3.724 3-079 4:151 5:439 4:143 2-297 2.092 1-780 4.741 November 3:360 3:441 2.634 3.775 4785 3:174 1.904 0.029 1.720 4:187 December - 3.832 3.288 2.569 3.955 6·084 3.142 1.981 1736 1.600 2:397 3-058 Fr. In. Fr. In. Inch. 9:530 2-995 3:537 3-120 36-140 134-121 27-664 139714 153-944 36919 21-331 20-686 18-649 35-977 2373. Frost, being derived from the atmosphere, naturally proceeds from the upper parts of bodies downwards; 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, turning what moisture is found there into a whitish substance like ice; and into standing water 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. 2574. 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. 2375. Hail is generally defined as frozen rain; it differs from it in that the hailstones for the most part 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 sometimes 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 or 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. 2376. Snow is formed by the freezing of the vapours in the atmosphere. It differs from hail and hoar frost, in being as it were crystallised, while 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 with 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. 2377. 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 contains 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. 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. 2378. 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 prodigious 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 neighbouring 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. 2379. The northern ice extends during summer 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 Mayer's Formula, and which is not however found to agree very exactly with what takes 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°. 2380. 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 westerly winds blow more upon an average in each month of the year than any other, particularly in July and August; the north-east wind prevails during January, March, April, May, and June, and is most unfrequent in February, July, September, and December; the north-west occurring more frequently from November to March, and less so in September and October than in any other months. 2381. Near Glasgow, the average is stated as follows: North-east Days. 104 North-west 2382. In Ireland, the prevailing winds are the west and south-west. 47 2383. The different degrees of motion of wind next excite our attention; and it seems almost superfluous to observe, that it varies in gradation from the mildest zephyr, which plays upon the leaves of plants, gently undulating them, to the furious tempest, calculated to inspire horror in the breast of the most callous. It is also a remarkable fact, that violent currents of air pass along, as it were, within a line, without sensibly agitating that beyond them. An instance of the fury of the wind being bounded "by a line" occurs in the hurricane of America; where its devastating course is often accurately marked in the forests for a great extent in one direction. 2384. Causes of wind. There are many circumstances attending the operations of the air, which we term wind, which serve for a basis for well-founded conjectures, and those, united to the result of daily observation, render the explanation of its phenomena tolerably satisfactory. 2385. It must be clear to the most common capacity, that as the rays of the sun descend perpendicularly on the surface of the earth under the torrid zone, that part of it must receive a greater proportion of heat than those parts where they fall obliquely; the heat thus acquired communicates to the air, which it rarefies, and causes to ascend, and the vacuum occasioned by this operation is immediately filled by the chill air from the north and south. The diurnal motion of the earth gradually lessens to the poles from the equator, at which point it moves at the rate of fifteen geographical miles in a minute, and this motion is communicated to the atmosphere in the same degree; but if part of the atmosphere were conveyed instantaneously to the equator from latitude 30°, it would not directly acquire the equatorial velocity; consequently, the ridges of the earth must meet it, and give it the appearance of an east wind. The effect is similar upon the cold air proceeding from the north and south, and this similarity must be admitted to extend to each place particularly heated by the beams of the sun. The moon, being a large body situated comparatively near the earth, is known to affect the atmosphere; and this, and the continual shifting of the point of the earth's surface over which the sun is vertical, to the west, are given as the causes of the tides and of the trade winds. The moon's revolutions, by pressing the atmosphere upon the sea, cause the flux and reflux which we call tides; it cannot, therefore, be doubted, that some of the winds we experience are caused by the moon's motion. 2386. The regular motion of the atmosphere, known by the name of land and sea breezes, may be explained by the effects of rarefaction: the air heated over the land rises up, because rarefied, and its place is supplied by the cooler air which flows in from the sea; this produces the sea breeze; at sunset, the equilibrium is first restored; but as the earth cools faster by radiation than the water, the air over it becomes cooler than that over the sea, especially if there be mountains in the vicinity; the air over the land then displaces the light air from the sea, and thus the land breeze is formed. Granting that the attraction of the moon and the diurnal movement of the sun affect our atmosphere, there cannot be a doubt but a westward motion of the air must prevail within the boundaries of the trade-winds, the consequence of which is an easterly current on each side: from this, then, it proceeds that south-west winds are so frequent in the western parts of Europe, and over the Atlantic Ocean. Kirwan attributes our constant south-west winds, particularly during winter, to an opposite current prevailing between the coast of Malabar and the Moluccas at the same period: this, he adds, must be supplied from regions close to the pole, which must be recruited in its turn from the countries to the south of it, in the western parts of our hemisphere. 2387. The variable winds cannot be so readily accounted for; yet it is evident, that though they seem the effect of capricious causes, they depend upon a regular system, arranged by the great Author of nature. That accurate and successful observer of part of his works, the celebrated Franklin, discovered in 1740, that winds originate at the precise points towards which they blow. This philosopher had hoped to observe an eclipse of the moon at Philadelphia, but was prevented by a north-east storm, that commenced at seven in the evening. This he afterwards found did not occur at Boston till eleven; and upon enquiry, he had reason to suppose, it passed to the north-east at the rate of about 100 miles an hour. The manner in which he accounts for this retrograde proceeding is so satisfactory, that we shall give it in his own words, particularly as his assertions are supported by recent observations, both in America and Scotland. He argued thus:-"I suppose a long canal of water, stopped at the end by a gate. The water is at rest till the gate is opened; then it begins to move out through the gate, and the water next the gate is put in motion and moves on towards the gate; and so on successively, till the water at the head of the canal is in motion, which it is last of all. In this case all the water moves indeed towards the gate; but the suc cessive times of beginning the motion are in the contrary way, viz. from the gate back to the head of the canal. Thus to produce a north-east storm, I suppose some great rarefaction of the air in or near the Gulf of Mexico; the air rising thence has its place supplied by the next more northern, cooler, and therefore denser and heavier air; a successive current is formed, to which our coast and inland mountains |