also most frequent, and next to them are the north-east. At Lancaster, a register kept during seven years exhibits the following average results:

:1

An abstract of nine years' observations made at Dumfries, gives

Dr. Meek's observations during seven years at Cambuslang, near Glasgow, show

The register from which this table is extracted shows the north-east wind to blow most frequently in April, May, and June, and the south-west in July, August, and September. The next table exhibits a view of the number of days during which the westerly and easterly winds blow in a year in different parts of the island, including under the term westerly, the north-west, west, south-west, and south, and taking the term easterly with the same latitude:

The result given by Professor Daniell from these and other observations, is, that in Great Britain, upon an average of ten years, the westerly winds exceed the easterly in the proportion of 225 to 140; and the northerly winds exceed the southerly as 192 to 173. The winds between north and east are almost invariably cold; those between south and west are warm; and those between north and west of a mixed character. But in our climate, and still farther north, two or three winds are often found blowing from different points within the distance of a few leagues.

CHAPTER XIV.

AQUEOUS ATMOSPHERIC PHENOMENA.

N addition to common air, a combination chiefly of the oxygen and nitrogen gases, united in different proportions, the atmosphere contains a mass of invisible vapour insinuated between the particles of the gases, and filtering through them, in a manner which may be compared to that of the diffusion of water through a sponge, or visible in the form of fogs and clouds. This vaporous atmosphere is the result of the ever-active agency of heat and electricity, which, by a process of marvellous subtilty and energy, evaporates the waters from the surface of the earth, and transfers them for a time to an aerial home.

The process is entirely untraceable by the eye of man, but its product appears in the clouds that are reared aloft in fantastic shapes, in the mists that occasionally shroud the landscape, the rain and the snow that come down from heaven, the dew glistening in the morning light, and the hoar-frost which adorns the forest with a beauty that throws the results of human artistic skill into insignificance. The words of the sacred writer are philosophically true: "He calleth for the waters of the sea, and poureth them out upon the face of the earth;" but the supply of humidity furnished to different countries varies prodigiously in its amount, and hence other differences as the consequence - barrenness here and fertility there a comparative solitude abandoned to the occupancy of the inferior orders of the animal creation, and a land studded with the homes of peasantry, the palaces of nobles, the halls of science, and the marts of commerce. Though we have spoken of the process of evaporation as untraceable by the human eye, yet that refers to the exhaling agency, for sensible evidence is frequently afforded that the ever-operating machinery is actually at work before us, in the visible exhalations we behold at early dawn, and in the calm evening of a summer's day. It is generally the case, however, that the formation of visible vapours takes place in the higher regions of the atmosphere, though at the earth's level, the metamorphosis of its waters into an invisibly vaporous state is constant, and is proceeding as powerfully when no outward sign of the process appears, and the air is perfectly transparent, as when a misty mantle, of feathery shape and texture, rests upon the lakes and rivers, and lies upon the surface of the valleys.

It is not merely from the great collections of water in oceans, lakes, and rivers, that evaporation takes place, but from the pasture grounds and forests; and Leslie makes the remark that even ploughed land will supply as much moisture to the exhaling fluid as an equal sheet of water. But the atmosphere is only capable of receiving a certain quantity of vapour in an invisible state, its capacity depending upon temperature, and being invariable in its extent at the same temperature. When all the interstices of the gaseous fluid are full, it is then said to be at its point of saturation, and any further supply of vapour becomes visible in the form of steam or mist. The lower the temperature, the greater the condensation of the air and the tightness of its particles, so that only a certain

amount of moisture can enter; but the higher the temperature, the greater the expansion, and the consequent capacity of a volume of the atmosphere to entertain the aqueous vapour. It has been computed, that a cubic mass of air measuring 40 inches each way, at a temperature of 68° Fahrenheit, can contain 252 grains of water, or, taking a cubic mass measuring 20 yards each way, at the same temperature, it will require 252 pounds troy of water to bring it to the point of saturation. Various causes contribute to accelerate or check the process of evaporation, but other things being equal, it will proceed most vigorously, the higher the temperature of the air above that of the surface upon which it acts, and be least active when the two temperatures are the same. The process is materially affected by the state of the air as to dryness and moisture, for water is rapidly evaporated by a stratum of dry air even when the temperature is low, whereas it is conducted tardily, if the atmosphere should contain much vapour, although the temperature may be high. The process also is powerfully promoted by the play of the winds, which bring the atmosphere into immediate and stronger contact with the moist earth and surface waters, and hence every one is familiar with the more rapid drying of the ground after rain, when the air is disturbed, than when it is still.

By the hygrometer, an instrument employed to ascertain the humidity of the atmosphere, as the name signifies, the measure of moisture, Professor Daniell calculated the average annual amount of evaporation in the vicinity of London to be 23-974 inches, proceeding at the following rate in the different months:

Thus, in that locality, evaporation is most active, and the largest amount of water is elevated into the atmosphere, in June, the reverse taking place in January. The annual evaporation from the whole surface of Great Britain is supposed to be equal to 32 inches of water. Now, water extended over the surface of the island to the depth of one inch, would amount to 309,696,038,000,000 cubic inches, which is equal to 1,116,931,402,691 imperial gallons, or 4,432,267,441 tons. If we multiply this quantity by 32, we have the prodigious sum of 141,832,558,752 tons of water, ascending in vapour every year from the face of the country. The power of the agency employed in this operation of nature must be tremendous; but equally for its utility does it command attention, as for its wondrous potentiality. For supposing this spontaneous evaporation to cease, the world being deprived of the elements that cause it, the heavens would drop no fatness; the springs would dry up, and the rivers be exhausted; the earth would soon be without any vegetation to adorn its surface, or any living creature to inhabit its wilds; the whole water of the globe accumulated in the ocean would overflow the land, and submerge its now fertile plains. In the temperate zone in general, with a mean temperature of 524°, the annual evaporation is estimated at between 36 and 37 inches; but in the torrid zone, where the temperature is much higher, the annual evaporation is greater. At Guadaloupe, one of the West India islands, it has been found to amount to 97 inches, and at Cumana, on the north coast of South America, to 100 inches.

The formation of visible vapours, and their aggregation in masses, take place generally in high regions of the atmosphere under the action of currents, in consequence of a decrease of temperature and a due supply of aqueous elastic vapour being present in those parts where clouds arise. It is easy to perceive that these two conditions, necessary to

the production of cloud-land, may be fulfilled in one stratum of the atmosphere and not in another, and hence the frequent diversity in the appearance of the sky, the clear blue fields and patches of ether alternating with visible vaporous structures. The clouds are supposed to consist of vesicular vapours, or minute globules of water filled with air, but there is great difficulty, even with the aid of this view of their structure, most probably correct, in explaining their suspension aloft, for the globules must be specifically heavier than the air by which they are upborne. The theory of ascending currents of heated air has been proposed by M. Gay Lussac to account for their position; and the retention of solar heat in the clouds themselves, buoying them up, and causing them to float, by M. Fresnel; but this is a point respecting which we are left without the guidance of any positive data. The clouds float at different elevations, but the higher we ascend, the drier the atmosphere is found, and the less loaded with vapours. "We shall not err much," says Mr. Leslie, "if we estimate the position of extreme humidity at the height of two miles at the pole, and four miles and a half under the equator, or a mile and a half beyond the limit of congelation." Dr. Dalton asserts that small fleecy patches of cloud are frequently from three to five miles in height, and such have been observed sailing above the most elevated peaks of the Andes, which rise 25,000 feet above the level of the sea; but other authorities claim for some visible clouds a still greater elevation. The height varies at different seasons of the year, and there is little doubt that it is much more frequently below than above a mile. Dalton gives a table from observations made by Mr. Crosthwaite of Keswick, who fixed marks on the side of Skiddaw, a mountain 1050 yards high, by which he was able to ascertain by inspection the height of the clouds when they did not exceed that of the mountain. During five years he conducted observations, three times each day, excepting a few intermissions which amounted only to missing less than a week per year. The table gives the number of times, in the respective months, that the clouds were at the height stated. The last column gives the number of times in which either the clouds were above Skiddaw, or there were no clouds at all.

5381

256542257536

901412241008 2

0400000000000

January
February
March
April

May
June
July
August
September
October
November

December 1

Total

10

42

62 179 374 486 416 367 410 518 409 2098

It thus appears, that, for 12 times that the clouds were from 200 to 300 yards high, in the month of January during the five years, there were 36 times in which they were from 1000 to 1050 yards high; and for twice that they were at the former elevation in the month of June, there were 34 times in which they were at the latter.

The forms assumed by the clouds are so infinitely diversified, as to render it apparently hopeless to attempt their arrangment in a few general modifications. But a classification has been made with some success, which reduces these varied aerial objects into seven genera, each of which is susceptible of such perspicuous description as to be readily recognised, and referred to its appropriate class and name. Mr. Luke Howard's ingenious

scheme is now universally adopted, which will be briefly given, placing Mr. Foster's English names beside the Latin nomenclature of the former writer.

Fig. 1. Cirrus-Curlcloud. This form of cloud exhibits light, flexuous, or diverging fibres, sometimes shooting out from a nucleus in all directions, resembling a lock of hair, or a crest of feathers. The name refers to this feature of its external character. It occurs, however, in parallel bars, or thread-like lines, spanning a vast extent of the atmosphere, the whole breadth of the sky being insufficient to show the extremities. Other lines also are occasionally presented, crossing these at right or oblique angles, as in a piece of network. In the former condition we have linear cirrus, and in the latter reticular cirrus. These are the cobwebs of the sky. They frequently appear stretching their white and delicate fibres between the dark and dense masses, as if spun to connect them, though really distinct and far separated. The cirri appear in the higher regions of the atmosphere, and are the most elevated of the clouds. Viewed from the summits of high mountains, while the traveller looks down upon other forms of cloud, he beholds these still above him, and apparently at as great a distance as when seen from the plains. The appearance of true cirrus, or curlcloud, is supposed to indicate variable weather; when most conspicuous and abundant, to presage high winds and rain; and when the streaming fibres have pointed in a particular direction for any length of time, the gale may be expected to blow from that quarter.

Fig. 5. Cumulus — Stackencloud. This modification of cloud occurs in the lower regions of the atmosphere, and is easily recognised. It is commonly under the control of the surface winds, and frequently exhibits a very magnificent appearance. It consists of a vast hemispherical or conical heap of vapour rising gradually from an irregular horizontal base, and increasing upwards. Hence the names, cumulus, a pile or heap, and stackencloud, a number of detached clouds stacked into one large and elevated fabric. Cumuli are the accompaniments and prognostics of fine weather. They begin to form soon after sunrise, from irregular and scattered specks of cloud, which then appear at a moderate elevation, and are the nuclei of the ultimate formations. As the morning advances the nucleus enlarges, or several coalesce, and early in the afternoon, when the temperature of the day is at its maximum, the cumulus attains its greatest magnitude. The cloud decreases as the sun declines, and is usually broken up towards sunset, rapidly separating into fragments, after the manner of its construction. The cumulus may be called the cloud of day, from the interval between morning and evening generally measuring the term of its existence. Its appearance considerably varies in the detail, and often exhibits a brilliant silvery light, and a copper tinge, when in opposition to the sun, indicating a highly electrical condition of the atmosphere.

Fig. 7. Stratus-Fallcloud. The former name, meaning a bed or covering, alludes to the position occupied by this cloud, immediately contiguous to the surface of the earth; and the latter to its origin, by the subsidence of vapour in the atmosphere. Unlike the cumulus, it eminently belongs to the night, appearing at eventide, reaching its maximum density soon after midnight, and commonly vanishing with the opening morn. This class of clouds comprehends all those fogs and creeping mists, which, in the calm evenings of hot summer and autumnal days, appear spread like a mantle over the surface of the valleys, plains, lakes, and rivers. The Roman poet held the nocturnal visits of the stratus to the lower levels to be an indication of continued fair weather:

"Then mists the hills forsake, and shroud the plain," -

a meteorological axiom, founded upon the popular experience, as true now as in the days of Virgil. The dissipation of the stratus does not always take place with the opening morn, no more than the wreck of the cumulus with the return of night, though in both