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Of the Varieties of the Fallcloud.
THOUGH most meteorological philosophers now concur in the general idea that dews and fogs are the result of vapour precipitated by the nocturnal decrease of temperature; yet the particular circumstances under which dew is formed in greater or less quantities, the time of night, and the kind of weather when it is most precipitated, and other facts relating to it, having been variously observed by different persons, have occasioned different views to be taken of their various causes. In this section, however, I shall confine myself to a few cursory observations on the varieties of appearance which the stratus presents.
Every body must have noticed the difference between the wet fogs (probably cirrostrati) which happen at all times of day, but often of a morning, and the white mists which wet nothing, but only leave dew in drops on the herbage,
* In Cornwall they amount to fine Rain almost; they call them the Pride of the Morning. Fine days frequently follow them.
which veil the meadows and vallies through a summer night, and ascend in the morning. As the temperature decreases in autumn, the stratus becomes thicker; the rays of the sun seem hardly able to overcome it, and it sometimes lasts throughout whole days; thus it gave rise in the minds of the ancients, whose organization led them to express physical facts metaphorically, to the fable of Phoebus and Python.*
In the neighbourhood of great cities these fogs, impregnated with numerous effluvia and smoke, have a yellow appearance which is explanable; but even in country places the yellow fogs of November extend over large tracts of land.
Dense fogs also happen sometimes, which appear suddenly, in different places; while at other times fogs continue for weeks together; such as that very thick and long fog, though
* Thus Phoebus, or the sun, is solicited by Cupid or love the vernal influence to court Daphne, and effect the fruits of love in summer's productions. He boasts to the little god of his recent victory over Python, that is, the fog spreading his pestiferous body over the meadows.
Qui modo pestifero tot jugera ventre prementem
Ovid. Met. II. 10.
one that did not extend very high, which in December, 1813, ushered in the long frost, that continued through January and February of the succeeding year. This fog seems by its topological history to have travelled from the West, Eastward, and Northward over our island. See some curious remarks about fogs, and particularly the extraordinary fog in France of 1783, in Bertholon, De l'Electricité des Meteors, Tom. II. Chap. 4, where the observations of different persons on this phaenomenon are duly noticed.
Of the Varieties of the Twaincloud.
WHETHER this cloud is formed with visible conjunction of different modifications, whether cumuli spontaneously assume its form, or whether it appears of itself previously, we must regard it as a stage towards nimbus. The very dense and black appearance of this cloud coming up with the wind, and just ripening into a Storm, must be familiar to every body. Where the Rain has actually begun to fall, the blackness
is changed for a more obscure and grey colour. This may be only the effect of the interposed water of the falling Rain; but if not, and if the nimbus be effected by an intense union of the watery particles, as I at present believe, the intense blackness of the previous cumulostratus must depend on some other principle. The mountains of this cloud, and its different appearances are mentioned in another place.
Of Nimbi which result from the visible Coalescence of distinct Clouds.
An artificial division may be made of nimbi AN into three kinds. Firstly; those which result from the visible coalescence of distinct clouds. Secondly; those which follow the interfusion of moisture between distinct clouds: and, thirdly; those which appear to form spontaneously in the air, without the precurrence of either of the above phaenomena. All these may, I think, be explained on the principle of the union of the differently electrified particles of which the clouds are composed.
If a cirrus, after it has ceased to conduct electricity, should receive from either mass of air, between which it may have been conducting, an electric charge, agreeably to the present theory it would loose its cirriform figure, and take on some other, perhaps a cirrocumulus, and by degrees would sink down towards the earth. Under such circumstances, it may come into actual contact with a cumulus rising from below by the upward propagation of diurnal temperature. Such a phaenomenon have I several times witnessed; and the result has been the sudden commixture of both clouds into a denser mass of nimbus, which has resolved itself into a gentle Shower, and all has disappeared; the union of the two clouds thus apparently effecting the destruction of both.
Such Showers, by visible inosculation, are of short duration: the process is soon finished; because the nimbus, thus formed, is circumscribed by dry air, and has no source of supply: and clearness returns, because the superfluous aqueous particles, or such as cannot be retaken into composition by the air, have come to the ground in Rain. When the circumjacent atmosphere has been moist, the process has been different, as described in the next Section