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it less capable, agreeable to M. Howard's theory, of containing so much aqueous gas in solution, which thus contributes to effect its own condensation into clouds: by which process there is produced again an increase of heat; and by this means the degree of temperature is in some measure restored.

"The theory of dew seems to be simply this: during the heat of the day a great quantity of vapour is thrown into the atmosphere from the surface of the earth and waters. When the evening returns, if the vapour has not been carried off in part by currents, it will often happen that more remains diffused in the general atmosphere than the temperature of the night will permit to subsist under the full pressure of the aqueous atmosphere. A decomposition of the latter then commences, and is continued until the general temperature and aqueous pressure arrive at an equilibrium, or until the returning sun puts an end to the process. The caloric of the decomposed vapour goes to maintain the general temperature; while the water is separated in drops, which, minute as they are, arrive successively at the earth in the space of a few hours. That the ordinary production of dew is by a real descent of water from the atmosphere, and not by decomposition of vapour on surfaces previously cooled (as in the experiment already mentioned) any one may readily be convinced by observing in what abundance it is collected by substances which are wholly unfit to carry off the requisite quantity of caloric for the latter effect."-Phil. Mag. Sep. 1803.

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In the foregoing chapter I have given a hasty and imperfect sketch of M. Howard's theory of the origin, suspension, and destruction of clouds. I shall next proceed to examine further the various appearances which the different modifications present in the progress of their formation, changes, and destruction; the influence which they appear to have on each other, and the connexion observable between their peculiarities and other atmospheric phaenomena. The reader will see how far these phaenomena are reconcileable with what has been already said in the above chapter. In investigating the causes of these changes, I have selected such few cases as appeared necessary to illustrate them, and for further particulars I refer to the Appendix at the end of the volume, and to my journal in the Philosophical Magazine. In this

journal, it may be said, I have been too minute in the detail of appearances. But where effects are intricate and complex, an accurate detail of them appears the only thing that can lead to a knowledge of their particular causes.


Of the Varieties of the Cirrus.

I HAVE already said that the Curlcloud was by no means uniform in its appearance; but, on the contrary, that it exhibited a very great variety of figure, both while it remained a determinate cirrus, and when passing to the other modifications. That these varieties are the effect of a variation in the cause of the cloud, cannot be doubted; many of them are attendant upon particular kinds of weather; and an accurate examination of them, compared with other coexisting phaenomena, seems likely to throw additional light on the nature of the peculiar office which the cirrus performs; namely, that of conducting the electric fluid.

When the weather is dry, the cirrus has more of a fibrous texture than when it is damp; and whatever may be its figure, whether


comoid, linear, or filiform, its extremities are always fine evanescent points. This is a fact very conformable to our present idea of its For when surrounded by dry air, which is an electric, there is not a free passage for communication; and the cirrus necessarily assumes that form which is best calculated for conducting, the evanescent terminations being probably points for the transmission of the fluid, and they are directed towards that part of the sky with which the electric communication is to take place.

On Friday, the 30th of August, 1811, the air being very dry, according to the hygrometer, the cirri were spread about in a lofty region; they were of a fibrous texture, one end terminating in transmitting points, the other frequently more massy; they passed on gently with the wind, in succession; by the evening none were to be Cumuli also, formed during the day in a lower atmosphere, moved along in the N. W. current, and likewise disappeared at night. The dryness of the air might be the reason why these two kinds of clouds did not unite and produce cumulostratus. At night cirrocumulus floated over from S. W.


In wet weather, when the air is damp, the

cirrus, which is seen in the intervals of the rain, is ill defined, and often of a sort of plumose figure; and it has less of the fibrous structure: this may be attributed to its being surrounded with moister air, which being a conductor, though an imperfect one, there is not the same necessity for the cirrus to be drawn out into fine transmitting points; as the fluid can fly off more generally from all parts of it. Cirri of this kind are generally of short duration, and have a great tendency to change into the other modifications; there is often a haziness in the atmosphere when they appear, and they are frequently soon followed by Rain. They seldom appear in fair dry weather; and if cirri, which have been previously fibrous, put on the plumose and indefinite character, a change to wet weather may be expected. All these are circumstances which corroborate the opinion, that the humidity of the circumjacent air is the cause of this kind of feature of the cirrus, and agree very well with the nature of the office already assigned to it.

I have almost always found the fibrous cirri to be accompanied by dry air. But there are rare exceptions to this rule. The upper air may, however, be tolerably dry, while the under air is moister, which may account for the occa

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