About a hundred yards below the junction, the temperature of the central part of the Danube was 59°, and here the quantity of mist was less. The evening of June 12th was cloudy, preventing radiation, and the temperature of the atmosphere remained till after dark higher than that of the river, when there was not the slightest appearance of mist. Similar observations were made on the Rhine, the Save, the Izonzo, the Po, the Tiber, and on the small lakes in the Campagna of Rome, and in no instance was there the formation of mist, but under the circumstances which have been detailed.

4. Cirrocumulus-Sondercloud.-This is a form of cloud of an intermediate nature between the cirrus and cumulus, and hence its Latin compound name. The cirrus, after having exhibited itself for a time, frequently passes into this modification, descending at the same time to a lower station in the atmosphere. Its parallel bars are broken into a number of small cumuli, of irregular shape, but generally orbicular, arranged in extensive beds, the component parts being quite distinct, or asunder, which explains the Saxon derivative title, the sondercloud. The previous appearance of the cirrus is not, however, necessary to the production of cirrocumulus, which often starts into existence independent of any other modification. The prevalence of this cloud in summer augurs an increase of temperature; and in winter, the termination of frost. Sometimes its different members are of very regular round form, dense structure, in close contact with each other, and arrarged on a curved base, in which state the cirrocumulus is commonly the natural harbinger of thunder-storms. In another variety, the small masses of cloud exhibit no uniformity of shape, and appear of a very light fleecy texture. Bloomfield's description of this cloud,

"The beauteous semblance of a flock at rest,"

aptly pictures its aspect at night in the presence of the moon.

5. Cirrostratus-Wanecloud.—These names point to the frequent origin and form of the cloud they indicate. It results from fibres of the cirrus waning or subsiding in the atmosphere, drawing closer to each other, and becoming arranged in horizontal strata. The cirrostratus exhibits several varieties:-a series of thin, inclined, and wavy streaks (fig. 1.); a row of short thick patches of cloud (fig. 2.); and a long horizontal sheet, very narrow in proportion to its extent, and attenuated at the edges (fig. 3). The appearance and prevalence of this cloud indicate wind, rain, or snow; and the second arrangement of it generally precedes storms, or occurs in the intervals of them. It is sometimes seen cutting the sun and the moon's disc with a dark line, or hanging over them like a thin hazy veil, one of the surest prognostics we have of a fall of rain or snow. Virgil in his Georgics gives it this interpretation:

"Or should his rising orb distorted shine

Through spots, or fast behind a cloud's dark line
Retire eclipsed; then let the swain prepare

For rainy torrents; a tempestuous air

Swift from the southern deep comes fraught with ill,

The corn and fruits to waste, the flocks to chill."

6. Cumulostratus - Twaincloud. This is the most magnificent form of cloud, as cirrocumulus is one of the most beautiful. It is formed either by two or more cumuli uniting together, or a single cumulus increasing laterally, so as to exhibit several vast hemispherical heaps overhanging the base. These mountainous masses form a multiple or twaincloud, and resting upon a common stratum are called cumulostratus. Nothing can be more imposing than the spectacle occasionally presented by these compound clouds, which the eye is disposed to contemplate as the architecture and home of giant spirits. The formation of cumulostratus takes place under different temperatures, and may precede a tempest of

snow, and a thunder-storm. It is the common herald of the latter, and may be seen rapidly forming during the calm which precedes a discharge of electricity, swelling to a stupendous magnitude, its protuberances, like the domes of an aerial city, shining with a strong silvery or golden light, finely contrasting with the darkness and density of its central regions. Borne by the currents of air, the cirrostratus is often conducted towards the summit of cumulostratus, and appears cutting through its whole extent.

7. Nimbus-Raincloud.—Any of the preceding modifications of cloud may so increase as to veil the sky completely, and put on an appearance of density, from which an experienced observer will augur rain. But they frequently dissolve without any shower, and no rain falls till another modification has been experienced, which commonly occurs in the case of cumulostratus. After exhibiting a great increase of density, and assuming a louring aspect, the blackness of darkness is followed by a lighter shade, evidencing a fresh disposition of the aqueous particles in the cloud, or the formation of nimbus, from which rain falls. This change may frequently be very distinctly observed when the cloud is over a distant spot; and the transition from considerable blackness to a gray obscurity is sure evidence that the shower has commenced, and may be expected to reach the locality of the spectator, should the wind be blowing in his direction, and the nimbus not be previously extinguished. Hence Virgil's reference to the husbandmen anxious to gather in the harvest :

"So while far off at sea the storm-cloud lours,
And on the darken'd wave its fury pours,
Mid crops unreap'd the hapless peasants stand,
And shuddering view its rapid course to land."

The nimbus-the least interesting modification of the clouds to the eye-is first in point of attraction when the rainbow appears upon its front. The precipitation of the aqueous vapours to the earth in the form of rain, is caused by contending aerial currents commingling saturated strata of different temperatures, promoting a condensation of the particles beyond what the air is capable of supporting, when the resulting mass gives out a portion of its moisture, which descends by its own gravity in rain, snow, or hail, according to the temperature of those regions of the atmosphere which it has to traverse. This is the last stage of an extensive pilgrimage which the evaporating forces in action constrain the waters of the globe to undertake through localities apart from its surface a pilgrimage in which there is no halt, and which some portions of the element are perpetually completing, commencing, and pursuing. How different and far apart the sites which are the starting and terminating points of the journey! Exhaled from the surface of a ruffled ocean or tranquil lake, the aqueous particles ascend invisibly into the upper air, where they are called into sensible existence by a change of temperature, and are built up into the variously-formed beautiful and majestic clouds. These are wafted by the atmospheric currents far apart from the scene of the ascension, change their shape and direction at the will of the winds, pass into a state of invisibility, and again emerge from it as warmer or cooler strata are encountered in their aerial flight, till, perhaps, a thousand leagues away from the spot where the liquid element assumed its vaporous form, that combination of circumstances occurs, which reduces it to its original condition, and deposition ensues upon some thirsty prairie or parching field.

The copiousness and energy of rain depend upon the amount of vapour in the atmosphere, and the gradual or rapid manner in which its particles are brought into mutual contact. We have a slow drizzle in the one case, and a violent shower in the other. The drops of rain vary in size, according to Leslie, from the twenty-fifth to the fourth of an inch in diameter. He remarks, that in parting from the clouds, their descent accelerates,

till the resistance opposed by the air becomes equal to their weight, when they continue to fall with an uniform velocity. The velocity bears a certain ratio to the diameter of the drops; those of a thunder shower, which are large, pouring down faster than those of an ordinary rain. The celerity of a small drop, th of an inch in diameter, he estimates at 11 feet per second, upon acquiring its uniform velocity; that of a larger one, 4th of an inch, at 334 feet. A great number of experiments have verified the remarkable circumstance, that a greater quantity of rain falls upon a low site than upon one a little elevated above it. Thus a rain-guage placed at the bottom of a hill, will collect a larger amount of water in a given time than another placed upon the summit. Dr. Heberden found that the annual depth of rain at the top of Westminster Abbey was 12.099 inches; at a lower altitude, on the top of a neighbouring house, it was 18.139 inches; and on the ground, in the garden of the house, it was 22.608 inches. M. Arago gives a similar result, from observations made during ten years at Paris. On the terrace of the Observatory the annual depth was 50-471 centimetres, or 19-88 inches; while thirty yards below, in the court of the building, it was 56.371 centimetres, or 22-21 inches. Comparing, however, an extensive tract of mountainous country with a low level district, the annual fall of rain in the former greatly exceeds that in the latter, though contrary to the natural presumption suggested by the fact, that the lower regions of the atmosphere are much more saturated with vapour than the upper. At Keswick in Cumberland a mountainous district the average annual depth of rain is 67.5 inches, while on the sea-coast it is not half that amount. On the Great St. Bernard it is 63.13 inches, and at Paris only 21.26. The description of Judea by the sacred writer, contrasting it with the flat lands of Egypt, though not intended to be philosophic, is in harmony with the teaching of science respecting the important part performed by mountains in the general economy of the earth: "For the land whither thou goest in to possess it, is not as the land of Egypt, from whence ye came out; but the land whither ye go to possess it, is a land of hills and valleys, and drinketh water of the rain of heaven." By arresting the course of the clouds, and producing a condensation of aqueous vapour when a warm current of air lights upon their cold summits, the elevations contribute to precipitate the moisture of the atmosphere, often amid a terrible display of electric phenomena - a blaze of fiery honours, and the echo of heart-thrilling sounds.

The annual amount of rain is the greatest between the tropics, and diminishes in general with the distance from the equator; but the number of rainy days is greater in high latitudes than in the torrid zone, owing to the showers in the latter region being more violent and prolonged. From north latitude 12° to 43° the mean number of rainy days is 78; from 43° to 46° the mean is 103; from 46° to 50° it is 134; and from 50° to 60° it is 131. By a comparison of observations made during twenty years at Salem and Cambridge in Massachusetts, with observations in twenty cities of Europe, it appears

that the number of rainy days is considerably more in the latter case, though the latitudes do not greatly differ.

In all latitudes the quantity of rain is greater in summer than in winter; but in the temperate zones, the showers are more frequent in winter, though less abundant, than at the opposite season. Thus at St. Petersburg the number of rainy or snowy days during the winter is eighty-four, and the quantity of rain that falls about five inches; but during the summer, with the same number of rainy days, the quantity that descends amounts to eleven inches. The discrepancy is immense between the tropics in the amount of rain in different months. At Bombay, the mean depth has been found by the pluviometer, or rain-gauge, to be 24 inches in June, and 1.26 inch in October. In extra-tropical climates the discrepancy is far less; but the last six months of the year appear to yield a larger supply of rain than the first. Forty years' observations made at London, give

The respective contributions of the different months with us are thus stated by Professor Daniell :

:

According to this table, the smallest depth of rain falls in February, and the largest in July. It is true of many places, though not perhaps generally, that the diurnal fall of rain is the greatest during those hours that the sun is below the horizon.

The average quantity of rain in the subjoined latitudes, with the mean temperature, is stated by Humboldt, as follows:

The occurrence of rain in tropical countries is a seasonal event, the year being divided into two periods of excessive drought and abundant showers, the sky remaining almost perfectly unclouded during the former season, and then becoming completely overcast at intervals during the latter. Districts situated north of the equator have their wet season from April to October, when the sun is in the northern half of the ecliptic, the reverse occurring on the south of the line. This is a remarkable instance of beneficial arrangement; for the rays of a vertical sun would be insupportable but for the screen of cloud which is coincidently expanded.

In some parts of the American continent, and in the West Indies, two wet seasons mark the year; but one is of much shorter duration, and has lighter showers, than the other. Two periods of rain are also mentioned in relation to Judea; the "first" or autumnal rains, which fall in seed-time, towards the close of October; and the "latter.” or spring rains, which fall in April, after the cold season. "I will give you the rain of

your land in his due season, the first rain and the latter rain, that thou mayest gather in thy corn, and thy wine, and thy oil." These two seasonal events were of vast importance to the Jews, though it is a mistake to suppose that rain seldom falls in Palestine except at those eras. It falls copiously then, and also occasionally through the winter months, its entire cessation being in the interval between May and October. Prominence is given to the two rains referred to, on account of their abundance, and especially the time of their occurrence, the success of the agriculturist depending in a great measure upon those plentiful showers. The periodical tropical rains do not fall for any considerable time without an intermission. After a fine morning, the clouds in general gather towards noon; the shower descends with great violence for four or five hours; and towards sunset, the sky clears, and remains cloudless through the night.

There is a considerable diversity in the amount of rain during the wet seasons in tropical countries, at different places, and in different years. In the ten years from 1817 to 1826 inclusive, at Bombay, the average annual quantity was 78.1 inches; but in the course of 1822 there fell 113 inches, while in 1824 the supply did not rise above 34 inches; and hence came famine and pestilence. At Bombay, also, the gauge has received as much as 16 inches of the 78 in the course of twenty-four hours; and while, there, the average annual quantity is as stated above, at Tellicherry, 12° north latitude, it is 116 inches, and in the delta of the Indus not more than 20 inches. There is great discrepancy between the amount at Calcutta and Benares; 72 inches at the former place, and only 46 at the latter. The greatest fall in those districts appears to take place on the eastern boundaries of the Bay of Bengal, where, in 1825, at Arracan, nearly 60 inches were registered in the month of July, and about 43 in August, from which, by a rough estimate, the annual amount is inferred to be not less than 200 inches. A more extraordinary quantity appears to fall in certain sites on the western continent, as in the forests of Guiana, where incessant rains of four or five months are no uncommon occurrence. The most remarkable instance of excessive rain is mentioned by Humboldt, upon the authority of Captain Roussin, who states that more than 160 inches have fallen at Cayenne in the single month of February. Erxleben mentions drops of rain at the equator occasionally an inch in diameter.

All countries however situated within and near the tropics are not thus favoured, as many parts of Africa, Arabia, and the coast of Peru are entirely rainless; and at Cumana, the annual quantity of rain does not amount to more than eight inches. The rainless regions seem to occur in two belts, one on each side of the equator, which would be consecutive but for the interruption of high lands, the nursery of the showers. The north belt commences in the old world on the west side of Africa. It includes the Sahara between 16° and 28° of latitude, but narrows as it proceeds easterly, extending from 19° to 27° on the banks of the Nile. In Arabia, it embraces the low coast, and part of the interior country, but its limits are not accurately known. From hence it passes through Beloochistan to the base of the Himalaya mountains, and beyond that range, there is the rainless table-land of Thibet. The southern belt occurs north of the Gareep or Orange river in South Africa, and includes extensive tracts in Australia. On the continent of America, rainless districts are found north and south of the equator, but the narrowness of the tropical parts of the continent, and the range of mountains that traverse it longitudinally, prevent the appearance of a showerless zone, as in the northern part of the eastern world. In both continents likewise the districts which have their periodical rains are subject to an occasional intermission, and become rainless for considerable intervals, the drought inflicting terrible suffering upon man and beast. Mr. Darwin speaks of the South American droughts being somewhat periodical, for upon comparing the dates of several, he found regular intervals of fifteen years between them. The