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Show 188 FORMATION OF CLOUDS. is barely visible, but after masses of cloud ·have been formed. Everybody who has looked at the sky must have seen the clouds "congregating," even when there was no wind but wind of their own making; and must have observed that, true to the law of that attraction which is the real cause of their formation, the little clouds always move towards and unite with the larger one. If the wind blows from a dry quarter, in the higher part of the air, the cloud is often swept away as fast as it forms; and if it be blown to a place where there is no such action on the surface as that which produced the cloud, it may be again dissolved by the air. But mornings when the disappearance of hoar frost denotes rain are generally calm ; and in those cases there usually is rain. Indeed, a moderate surface wind, one of those "unhearty'' winds which we call "raw." and which hiss in the crevices like scotched snakes, rather brings on than retards the rain; as wind always increases evaporation-even those winds that we call "moist," dry more than air at the same degree of saturation with moisture, but at rest. When the heating cause is local and confined, the result is not rain but fog. In the evening, the land, especially where it is bare and dry, cools much sooner than the water; and as it is the change of temperature, and not the absolute temperature that produces the change of evaporation, vapour then gathers over the pools and marshes, and the courses of the rivers; and among bare hills with deep valleys, and lakes and rivers, the fog is often seen white and :lense, in the hollows, as if some white fluid had been poured into them. City fogs, such as the fog of London, which IS at times very annoying, and always very offensive, are owing to a similar cause; only in the case of these that cause is in the city. In the early morning, when the Droduction of fog has been lessened by t~l6 LONDON l<'OG. 189 slackening oithe fires during the h;)urs of rest, and the upper air, which may be very dry and tranquil without the limits of the city heat both upwards and laterally, may have melted the fog of the preceding day, the air may be moderately clear. But when the halfmillion of fires are lighted, and send up their heat, the whole moisture of the surrounding air is poured over the city; and that, mingling with the evaporation from the city itself, becomes so dense, that the charcoal, and the nitrate of ammonia, and all the other matters which, at ordinary times, the air disperses in great part, float mixed with the watery vapour, and produce an atmosphere approaching as nearly to the consistency of a quagmire in the air as it is perhaps possible to obtain. But unpleasant and inconvenient as the London fcg is, and much as it prevents all means of observation, there is still something in it worthy of attention to the observer of nature. The fog is a natural production, though some of the elements of it are brought together by artificial means; and thus, though they be somewhat dismal charms, it has still some of the charms that belong to all natural phenomena.. It is curious.to find a sort of twilight representatiOn of London m that very substance which completely hides London itself; and yet such is the case. It is not to be understood that the wards, and cities, and boroughs which compose the metropolis, are as well represented by their several fogs as they are by other means ; but still they are represented by these. The air over London moves upwards and downwards with the tide of the river; and over rivers of ~uch m~gnit?de the light winds are more frequently m the direction of the tides than in the cross direction. ,-rhe light winds that accompany the fog, !hough they barely reach the streets, and are not mdeed very perceptible when so little can be seen, are usually from the east. Hence, if the tide is |