of the fog less perfectly than damp ground. Here and there, like huge hummocks, the fog stands out denser than the surrounding parts, marking the vicinity of some great brewery, or a more than usually dense population, where houses are thick together, and smoke issues from every chimney. Yonder, where the fog stands so high, the subsoil is of clay, from which the moisture escapes with difficulty; in other parts, where the fog is low, the ground is of sand or gravel, through which the wet filters easily. The neighbourhood of the parks and the wider streets, and larger houses, are also indicated by the thinner texture of this vaporous ocean. "London fog differs essentially from clouds and mists and dew properly so called. Clouds are formed in the sky at some height in the air; but fog is an earth-cloud formed at the surface of the earth, and seldom ascending many feet above it. Clouds and mists and dew are all wet and damp and uncomfortable; but London fog is perfectly dry, and enfolds the city as in a garment. Dew is formed by the cooling down of the earth's surface below the temperature of the air sufficient to condense the moisture of the air upon it in liquid drops; whereas a fog is formed by a cold upper stratum of air coming in contact with a lower and warmer stratum, and partially condensing the vapour wherewith it is charged. "But London fog consists of something more than vapour of water partially condensed. Every house contributes a number of little fiery volcanoes, which are busy all day long supplying to the air vapour of water, sulphur, carbon, as well as sulphurous, nitrous, and pyroligneous acids, and some other matters, all of which become to a great extent mixed with the vapour of the air. It must further be noticed that the various constituent parts of the fog, being formed at the surface of the earth, are in the same electrical state as the earth itself, and hence (the air being a non-conductor) they repel each other, and are repelled by the earth, and thus remain suspended. This also furnishes the reason why the fog is dry, and does not adhere to any surface. 66 A very moderate wind is sufficient to disperse a fog; but an increase of moisture at the surface without rain, or an increase of temperature without wind, generally makes the A LONDON FOG. 505 fog more dense, because the heat increases the evaporation of the moistened surface. Thus it often happens that London is free from fog during the middle of the night and the early hours of the morning, but as soon as the fires are lighted the fog begins to appear. It sometimes clears away about noon, and becomes more dense towards evening, as the gas and other lamps are lighted, but the chief reason probably is, that after sunset the natural fall of the atmosphere presses the fog nearer to the earth's surface, and consequently makes it more dense. The same cause also sends the fog down the chimneys which have no fire, producing in the apartments a strong odour of soot, or a smell as of something burning. By night this has often caused anxiety. "The London fog has a peculiar tidal motion, tending, however, rather down than up the river, towards the marshes, where it performs a peculiar service to our markets in supplying them with much finer wild fowl than are produced in districts not visited by fogs. These birds feed only during twilight, and as the fog extends the twilight over nearly the whole day, they are constantly feeding. Thus when the birds are caught in clear weather after a fog they are found to be in very fine condition. "The appearance of fogs in different cities is frequently simultaneous. At the time a great fog prevailed in London, Dublin was visited by one of equal density. Fogs have been known also to precede a strong westerly wind at Manchester, London, and Paris, and by nearly the same short interval of time at each station. "The poet Crabbe gives a very true description of a fog at the sea-coast. "When all you see through densest Fog is seen, CHRONICLE OF THE SEASONS. LEAFLESS TREES. To the lover of nature, the trees in casting off their summer garments of rich leaves only reveal to him fresh objects of beauty and delightful study in the marvellous construction of their noble skeletons, in the exquisite tracery produced by the intermingling of myriad branches and delicate twigs, in the lavish variety of character stamped upon each separate species of tree, and upon each individual of that species. "I do not propose," says Ruskin in his "Modern Painters," "to examine the characteristics of each tree; it will be enough to observe the laws common to all. First, then, neither the stem nor the boughs of an oak, elm, ash, hazel, willow, birch, beech, poplar, chesnut, pine, mulberry, olive, ilex, carob, or whatever the tree may be, taper, except where they fork. Wherever a stem sends off a branch, or a branch a lesser bough, or a lesser bough a bud, the stem or the branch is, on the instant, less in diameter by the exact quantity of the LEAFLESS TREES. 507 branch or the bough they have sent off, and they remain of the same diameter, or if there be any change, rather increase than diminish, until they send off another branch or bough. This law is imperative, and without exception; no bough or stem, or twig, ever tapering or becoming narrower towards its extremity by a hair's breadth, save where it parts with some portion of its substance at a fork or bud, so that if all the twigs and sprays at the top and sides of the tree which are, and have been, could be united without loss of space, they would form a round log of at least the diameter of the trunk from which they spring. "But as the trunks of most trees send off twigs and sprays of light underfoliage, of which every individual fibre takes precisely its own thickness of wood from the parent stem, and as many of these drop off, leaving nothing but a small excrescence to record their existence, there is frequently a slight and delicate appearance of tapering caused in the trunk itself; while the same operation takes place much more extensively in the branches; it being natural to almost all trees to send out from their young limbs more wood than they can support; which, as the stem increases, gets contracted at the points of insertion, so as to check the flow of the sap, and then dies and drops off, leaving all along the bough, first on one side, then on another, a series of small excrescences sufficient to account for a degree of tapering, which is yet so very slight, that if we select a portion of a branch with no real fork, or living bough to divide it, or diminish it, the tapering is scarcely to be detected by the eye; and if we select a portion without such evidences of past ramification, there will be found none whatsoever. "But nature takes great care and pains to conceal this uniformity in her boughs. They are perpetually parting with little sprays here and there, which steal away their substance cautiously, and where the eye does not perceive the theft until a little way above it feels the loss; and in the upper parts of the tree, the ramifications take place so constantly and delicately, that the effort upon the eye is precisely the same as if the boughs actually tapered, except here and there where some avaricious one, greedy of substance, runs on for two or three yards without parting with anything, and becomes ungraceful in so doing." SHOOTING STARS. The most important of the periodic fall of Shooting Stars occurring in this month, we extract from "Cosmos an interesting passage relating to them. "Shooting-stars," says Humboldt, "fall either separately and in inconsiderable numbers, that is sporadically, or in swarms of many thousands. The latter, which are compared by Arabian authors to swarms of locusts, are periodic in their occurrence, and move in streams, generally in a parallel direction. Amongst periodic falls, the most celebrated are known as the November phenomenon, occurring from about the 12th to the 14th of November, and that of the festival of St. Lawrence (the 10th of August), whose 'fiery tears' were noticed in former times in a church calendar of England, no less than in the old traditionary legends, as a meteorological event of constant recurrence. Notwithstanding the great quantity of shooting-stars and fire-balls of the most various dimensions, which according to Klöden, were seen to fall at Potsdam, on the night between the 12th and 13th of November, 1822, and on the same night of the year in 1832, throughout the whole of Europe; from Portsmouth to Orenburgh on the Ural River, and even in the Southern Hemisphere, as in the Isle of France; no attention was directed to the periodicity of the phenomenon, and no idea seems to have been entertained of the connection existing between the fall of shooting-stars and the recurrence of certain days, until the prodigious swarm of shooting-stars which occurred in North America, between the 12th and 13th of November, 1833, and was observed by Olmsted and Palmer. The stars fell, on this occasion, like flakes of snow, and it was calculated that at least 240,000 had fallen during a period of nine hours. Palmer, of Newhaven, Connecticut, was led, in consequence of this splendid phenomenon, to the recollection of the fall of meteoric stones in 1799, first described by Ellicot and myself, and which, by a comparison of the facts I had adduced, showed that the phenomenon had been * Bohn's translation, vol. i. p. 111, et seq. |