The only Indications of rain or fair weather upon which any reliance may be placed are those which have been noticed by the late Sir Humphrey Davy, in his Salmonia;' and as his explanations are founded on physical conditions, a brief statement of them may with propriety be introduced in this place.
One of the speakers In the Dialogue inquiring why the clouds In the west being red, with a tinge of purple, should portend fair weather, is answered that the air, when dry, refracts more of the red and heat making rays than when moist ; and as dry air is not perfectly trans parent, those rays are reflected in the horizon. It Is added that a coppery or yellow sunset foretells rain; but that as an indication of approaching wet weather, nothing is more certain than a halo round the moon, since it is produced by precipitated water : the larger the circle Is, the nearer are the clouds ; consequently the more ready to descend in rain.
In explaining why a rainbow in the morning betokens rain, and one In the evening fair weather, It is stated that the bow can only be seen when the clouds depositing the min are opposite to the sun; thus in the morning the bow Is In the west, and in the evening In the east : and as the rains iu this country are usually brought by westerly winds, a bow in the west indicatea that the rain is coming towards the spectator; whereas a bow in the east indicates that the rain is passing away from him.
The indications of fine weather from swallows flying high is ex plained by stating that the insects on which these birde feed delight to fly in a warm stratum of air ; but warm air, being lighter than that which is moist, occupies a higher part of the atmosphere, and, there fore the birds then find their prey in the upper regions. On the contrary, when the warm air is near the surface of the earth, the insects and birds are there also ; and then, as the cold air from above de ..ends into it, a deposition of water takes place. The opinion that sea-birds come to land In order to avoid an approaching storm is stated to bo erroneous; and the cause assigned is that, as the fish upon which the birds prey go deep into the water during storms, the birds come to land merely on account of the greater certainty of finding food there than out at sea.
It may be observed hero, that the kind of cloud which is designated cirrostratus [Ctouns), in which, when the particles of water composing it are to a state of approximate coalescence into drops, the halo is formed, is almost always followed by a depression of temperature in the atmosphere, and by wind or rain. For indications of the weather, which. are afforded by the oscillations of a mercurial column, see Basionaren ; and for those which precede cyclones, or revolving atones, oee that subject in the article Viso.
The observations of Principal Forbes (stated in the article l'aroutt, OPALESCENT) have shown that the red colour of the clouds is refcrrible to a different cause from that to which It is ascribed by Davy, in the citations above. But the red evening and gray morning have been
regarded as the surest and most consistent signs of fine weather. They would naturally be observed from the earliest periods, and accordingly they appear to be the most ancient of prognostics, having been recorded in the verses of the Greek poet Aratua, who was contemporary with Euclid ; in the New Testament (Matt. xvi. 2, :1); and in one of our most familiar b roverbs. The purple tinge alluded to, according to Principal Forbes, probably arises from a mixture of the reflected blue of the pure sky, which is always present when purple is seen, with the yellow-orange of the opalescent vapour.
"The modified hues of the sky, and of the sun and moon near the horizon, have, for so many ages and In so many countries, been regarded as the surest indications of atmospheric changes, that we cannot doubt that it is to the variety of conditions in which vapour exists in the air, more or lees nearly condensed, that these phenomena are due. Hum boldt describes the colour and form of the sun's disc at setting in tropical regions, as the most infallible prognostic, and elsewhere ascribes these variations 'to a particular state of the vesicular vapour. Since the red steam [opalescent vapour] occurs only during the critical stage of its partial condensation (and perhaps conversely during evapo ration), it is evident that it must correspond to a critical state of diffused vapour in the atmosphere. . . . . Every accurate observer of nature in alpine countries will confirm me in stating that fine weather is almost invariably accompanied by the formation of dew on exposed surfaces, and by the progressive depression of the moister strata, until at length visible fogs are formed in the bottom of the valleys, and espe cially over water. (For the reason why over water, see Davy This is the surest sign of a following fine day in mountainous regions. . . . . The inflamed appearance of the morning sky, considered indi cative of foul weather, is, I have no doubt, o sing to such an excess of humidity being present that clouds are actually being formed by con densation in the upper regions, contrary to the direct tendency of the rising sun to dissipate them, which must therefore be considered as indicating a speedy precipitation of rain." (` PhiL Mag.,' series 2, vol. ay., pp. 34-37.) Principal Forbes expresses a strong doubt as to the formation of any "vesicular" vapour in the process of the condensation of atmo spheric vapour; on which it may be remarked that the existence of such vapour (that is, in fact, of vesicles of water produced by the con densation of vapour) under any circumstances is altogether hypo thetical, as noticed in considering the nature of the spray of WATEII• PALLS. According to observation, when vapours become liquids, minute globules geometrically solid are formed: the globules of dew are such.