cases this is the general rule. It sometimes survives throughout the entire day, or maintains a successful conflict with the solar beams to an advanced period of the morning, accumulating first in heaps, then separating from the earth, and losing its continuity,

Loch Achray.

before retiring from the field. The
effect is striking, when from an
eminence which commands a view of
an extensive plain or valley, we see
this gossamer curtain of the night
resting upon the surface, gradually
rent and torn by the action of the
sun's rays, reflecting as it disappears
their golden hue. Many of the most
felicitous images of poetry are derived
from this source, as in Ossian :
"The soul of Nathos was sad, like
the sun in a day of mist, when his
face looks watery and dim;
" and
again, when two contending factions
are silenced by Cathmor:-"They
sunk from the king on either side,

like two columns of morning mist, when the sun rises between them on the glittering rocks."

The stratus is occasionally seen under peculiar and striking circumstances, extending over the surface of a sheet of water, without passing the boundary of its banks. Thus a lake or river will exhibit a white cloud of visible vapour resting upon it, from which the adjacent land is perfectly free. When in the neighbourhood of Loch Achray, well known to the readers of Scott,

"The minstrel came once more to view

The eastern ridge of Ben-venue,

For, ere he parted, he would say
Farewell to lovely Loch Achray-
Where shall he find, in foreign land,
So lone a lake, so sweet a strand?"

at the close of a calm and warm September day, the expanse was nearly covered with a beautiful stratum of mist, while the atmosphere of its borders presented no trace of visible vapour. Mr. Harvey repeatedly observed a similar cloud hovering over the stream which supplies Plymouth with water, whose boundaries on a calm night would exactly coincide with the banks of the stream, however winding and irregular its outline. Sir Humphry Davy thus explains this curious phenomenon: "All persons who have been accustomed to the observation of nature, must have frequently witnessed the formation of mists over the beds of rivers and lakes in calm and clear weather after sunset; and whoever has considered these phenomena in relation to the radiation and communication of heat and the nature of vapour, since the publications of MM. Rumford, Leslie, Dalton, and Wells, can hardly have failed to discover the true cause of them. As, however, I am not aware that any work has yet been published in which this cause is fully discussed, and as it involves rather complicated principles, I shall make no apology for offering a few remarks on the subject to the Royal Society. As soon as the sun has disappeared from any part of the globe, the surface begins to lose heat by radiation, and in greater proportions as the sky is clearer; but the land and water are cooled by this operation in a very different manner: the impression of cooling on the land is limited to the surface, and very slowly transmitted

to the interior; whereas in water above 40° Fahrenheit, as soon as the upper stratum is cooled, whether by radiation or evaporation, it sinks in the mass of fluid, and its place is supplied by water from below, and till the temperature of the whole mass is reduced to nearly 40° Fahrenheit, the surface cannot be the coolest part. It follows, therefore, that wherever water exists in considerable mass, and has a temperature nearly equal to that of the land, or only a few degrees below it, and above 40° Fahrenheit at sunset, its surface during the night, in calm and clear weather, will be warmer than that of the contiguous land; and the air above the land will necessarily be colder than that above the water; and when they both contain their due proportion of aqueous vapour, and the situation of the ground is such as to permit the cold air from the land to mix with the warmer air above the water, mist or fog will be the result." He thus accounts for the formation of mists over water, by the difference in the rate of cooling, in the absence of the sun in fluid and in solid bodies. The atmosphere reposing on the water continues warmer after sunset on a clear night than the air of the adjoining land. It obtains also a greater supply of moisture from its position over an aqueous surface, which is condensed into visible vapour by the colder air of the land intermingling with it. On descending

the Danube during the three nights of June 9th, 10th, and 11th, 1818, Sir H. Davy observed, that the mist regularly appeared over the river in the evening, when the temperature of the air on its banks was from 3° to 6° lower than that of the stream, and that it as regularly disappeared when the temperature of the air on the banks surpassed that of the river. Below Passau, where the milky blue waters of the Inn, and the perfectly pellucid Ilz join the green current of the Danube, he found their temperature and that of the atmosphere on shore, with the appearance of the rivers, as follows, at six o'clock in the morning :

Temp. of the Air
on the Banks.

54°

Temp. of the Rivers.

Danube, 62°

Inn, 5610

Ilz, 56°

State of the Rivers.
Thick fog on the whole breadth.
Slight mist.

Haziness.

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.

2. 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 arranged 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.

3. 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; a row of short thick patches of cloud; and a long horizontal sheet, very narrow in proportion to its extent, and attenuated at the edges. 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."

4. 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