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Of the Publishers of this work may be had, by the same Author,

1. APATOT AIOEHMEIA, Arati Diosemea, Notis ex Collatione
Scriptorum illustravit Thomas Forster, &c.

2. OBSERVATIONS ON THE BRUMAL RETREAT OF THE SWALLOW,
&c. 3d edition, enlarged.

3. ON PERIODICAL AND ATMOSPHERICAL DISEASES.

4. SKETCH OF THE PHYSIOLOGY OF THE BRAIN, &c.

5. NATURAL HISTORY OF THE GENUS HIRUNDO, &c.

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OF M. HOWARD'S THEORY OF THE ORIGIN AND

MODIFICATIONS OF CLOUDS.

A CLOUD is a visible aggregate of minute particles of water suspended in the atmosphere. In the more extensive signification of the word, smoke and all the visible effluvia of volatile substances may be considered as clouds; meteorologists have, however, confined this term to aqueous particles.*

* Our English word Cloud is derived of the Anglo-Saxon verb hliðan or Lehlidan, tegere, to cover; from the same verb came glade, blot, lot, and lid. In like manner, the Latin nubes, and its diminutive nebula, came from the Latin verb nubere; and from the same verb is derived nupta. So the Greek νεφος and νεφελη from νεφεον. It need hardly be observed here, that all that words can do, is to express some of

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Before I speak of the origin, suspension, and varieties of clouds, and of their destruction by rain, some preliminary observations will be necessary. Aqueous particles, and other volatile substances, may be either diffused in the air, or may be dissolved in it. But diffusion and solution are things quite different from chemical combination.

A cloud may either be so diffused as to cease to be visible as an aggregate, or it may be taken into solution by the air: in the former case, a hazy turbidness; in the latter, an additional clearness of the sky; would probably be the consequence.*

the qualities of the thing they represent; they serve merely as hints for the production of ideas.-See TOOKE'S ETA Πτεροεντα, 4to. vol. ii. 196.

* A cloud may be the consequence of vapour, upraised into the air, and afterward more condensed into visible particles, by an alteration either in the temperature or pressure, whereby the air cannot hold so much vapour in solution as before. Some recent discoveries have, however, led to a supposition that, under particular circumstances, the air itself may be decomposed so as to deposit water, which may again be taken up by the air. Thus we come back again to the old opinion of Aristotle:-Ει δη γίνεται υδωρ εξ αερος, και αήρ εξ ύδατος, δια τινα σοταίτιάν ου συνιστάται νεφη κατα τον άνω TOTOY, &c. - Meteor. lib. i. cap. 3.

I speak first of clouds, because in the observance of the varying countenance of the sky, as M. Howard terms it, and of its connexion with atmospheric changes, consisted the popular meteorology of the ancient agriculturists, who were chiefly concerned to inquire

Quo signo caderent Austri, quid sæpe videntes

Agricolae propius stabulis armenta tenerent. And the accuracy of their observations, with respect to prognostics of the change of weather, have been verified by those of more modern meteorologists. It is obvious, however, that the ancients wanted definite terms whereby to express the peculiarities observable in clouds and other atmospheric phaenomena; a deficiency which has been in some degree supplied by the moderns, and particularly by M. Howard, whose theory of the formation and destruction of clouds appears, as far as I am capable of judging, to be extremely accurate in most particulars. As it will be necessary for me to have perpetual reference to this theory, and as I shall always use the terms which he has adopted, it will be proper to present the reader with the substance of it, as nearly as I can recollect it, with such additional observations as I have been enabled to make since, together with

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