SECTION III.

Of the Cumulus or Stackencloud. Pl. I. Fig. 3.

CVMVLVS. Def. NVBES CVMVLATA DENSA SVRSVM

CRESCENS.

THE Cumulus is a convex aggregate of watery particles increasing upwards from a horizontal base. It is commonly of a dense structure, formed in the lower atmosphere, and moving along in the current of wind which is next to the earth. Its first appearance is generally a small irregular spot, which becomes the nucleus on which it forms. This increases in size, preserves a flat horizontal base, and assumes more or less of a conical figure. Cumuli Cumuli vary in shape and dimensions, according to peculiarities in the operation of the causes which produce them. Sometimes they are pretty well defined hemispherical masses; at others, they rise into mountains, ranged in one plane, their silvery summits presenting a beautiful appearance. In particular kinds of weather, I have seen cumuli of a sort of tuberculated structure. Before rain, they increase very rapidly, descend lower in the

atmosphere, and become fleecy and irregular in their appearance, with their surfaces full of protuberances. In changeable weather, they partake of the vicissitudes of the atmosphere, and evaporate almost as soon as formed, or quickly change into other modifications. The quick formation and subsiding of this cloud may be observed too in fair weather, with easterly and varying breezes, particularly in spring; but, in fair and settled weather, they keep pace in some measure with the diurnal temperature, they form soon after sunrise, arrive at their maximum in the middle of the day, and become very convenient skreens to intercept the rays of the sun; and they subside in the evening. It was this circumstance which probably led to a conjecture of the particular cause of their production, which appears as follows:-The sun's rays warming first the surface of the earth, and their radiation causing warmth to be propagated upward, this warmth converts water on the earth's surface into vapour, which rises and exerts its elastic force on that which the nocturnal decrease of temperature had not decomposed, and which therefore remained diffused. The latter, in passing through the atmosphere, to give place to that from below,

changes its climate, arrives in a colder air, and is thereby decomposed and thrown into a state of visible cloud. The simple attraction of aggeration may perhaps cause the watery particles to collect in a mass, while their being similiarly electrified may render them mutually repulsive, and prevent their uniting to become rain. The cumulus preserves its plane base, because it floats on the vapour plane, or at that precise elevation at which the air has as much water in solution as from its quantum of heat and pressure from above it is able to contain. Whether the conical form of this cloud is to be attributed to the attraction of aggregation alone, or whether something particular in its electric state may also be concerned, has never, I think, been determined. The variation of its figure, according to different states of weather, seems to favour the latter supposition.

The cumulus, then, may either evaporate, change into the other modifications, or, by inosculating with any of them differently electrified, may form the cumulostratus, and ultimately the nimbus, hereafter to be described.

* This, however, is doubtful, as I mention in the account of the stratus.

SECTION IV.

Of the Stratus or Fallcloud. Pl. II. Fig. 4.*

STRATVS. Def. NVBES STRATA AQVAE MODO EXPANSA, DEORSVM CRESCENS.

.

THE stratus is the lowest of clouds; its under surface usually rests on the earth, or on the water. It may properly be called the cloud of night, as it frequently makes its appearance about sunset, and disappears soon after sunrise. When ascending in the atmosphere, it often seems at a certain elevation to take the irregular hemispherical form, and become a cumulus. It comprehends what we usually call fogs and mists, which in fine summer evenings are seen to ascend in spreading sheets from vallies, lakes, and fields. And which in autumn and winter sometimes continue throughout the day as dense

* This cloud is also called Evencloud, and by the Germans Abendwolke, thus in the old popular ballad: Morgen fürht die junge Braut

Heute zum Altare,

Eh die Abendwolke thaut,

Ruht sie auf der Bahre.

fogs. It must be remembered, however, that all fogs are not strati: some appear to be of the modification of cirrostratus. Of the latter kind are generally the wet mists, which moisten every thing on which they alight.

In speaking of the cumulus, I have represented the manner in which elastic vapour may rise into the air, on the accession of diurnal temperature. As the sun sinks, the heat also is diminished, and the lower atmosphere becomes cooler than that above. The air, no longer capable of containing so much vapour in solution as when it was warmer in the day, may deposit it in minute particles of water, which may fall in the form of mist or stratus. In the evening, too, the under atmosphere being as cold, or perhaps colder, than the upper, the vapour plane is not preserved, and cumuli by degrees may sink down in dew. Under these circumstances, they appear often to evaporate.* This vespertine subsidence of the cumulus is a circumstance which induces me to believe that its diurnal existence, as an aggregate, is not merely the result of the attraction of aggeration. Its sub

* For further observations respecting the nocturnal evaporation of clouds, the decomposition and recomposition of the air, etc. I refer the reader to the next Chapter.