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,

66

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

The stratus is occcasionally 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 Humphrey 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 bardly 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

H H

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: