« ПредишнаНапред »
currents of air, which must be supposed to prevail in the higher regions of the atmosphere. Or again, if we consider its vast bulk, which notwithstanding still represents the form of the mountain, (corresponding with its elevations and depressions) so that the boldest head of CrossFell, which is somewhat to the northward of the middle part of its range, is still distinguishable even in its Helm; for such is the name of that heap of vapour from whence the wind, of which we are treating, issues. When this heap first gathers upon the hill, there is seen above it a black streak of cloud continually flying off, and continually fed from the white one, which is called the real Helm; this is called the Helm-bar, from its being supposed to bar or obstruct the winds that burst forth upon the vallies beneath as soon as it wholly vanishes; its direction is parallel to that of the white cloud, and it seems in continual motion as if boiling, or at least agitated by a violent wind; and indeed the wind, which really does follow its removal, is sometimes prodigiously violent; varying with respect to the extent of territory which it affects in proportion to the force and direction of what I shall here call the Real Winds. Sometimes when these are its direct antagonists, and in full force, it does not reach farther than two or three miles ; nor do I know that even without such impediment it ever extends farther than thirteen or fourteen, being interrupted in its progress by the vis inertiæ of the air at large, or by some cause arising from the impulse of contrary currents. However, though it always bears a certain proportion to the force and direction of the real winds, its own intrinsic force is not always equal, nor is it found so even at the foot of the hill, where, on account of the shortness of its course, the action of contrary currents or the resistance of the air can not be supposed to have had any material effect in changing the degree of its power, or of interrupting its progress. It may, however, ,
be remembered as a truth, that near the base of the mountain it is at times excessively strong, bearing almost every thing before it, though at a distance of a very few miles not felt at all.
“ Such is the Helm-Wind generated in that enormous cloud, which like a helmet covers the summit of Cross Fell. It is here particularly favoured by circumstances ; for on one side there is a plain of above thirty miles in breadth in some places, and on the other no hills to rival that from whence it comes. This wind is not much taken notice of in natural history; yet the Dutch by the iron chains, with which they are obliged to moor their ships at the Cape of Good Hope, bear ample testimony to the fury of such an one. It hath been met with by late voyagers in the South Seas; it is said to have been felt in the straits of Gibraltar; and I doubt not but mariners and travellers have found it in many other places, though they may not have observed it with care, or may have given it different names. I apprehend that the landbreeze in the West Indies, though less violent and more regular, is similar to it; and I doubt not that there may be a helm-wind from almost every hill covered with a cloud in certain kinds of weather, though the resistance of similar winds from neighbouring hills may prevent its being taken notice of.
“It may be remarked of this wind, that it generally blows from Cross Fell longest in the spring, when the sun has somewhat warmed the air beneath; and does not cease till it has effectually cooled it. Thus it sometimes continues for a fortnight or three weeks, which I consider as a peculiarity of the Helm-wind of Cross-Fell."
“THE BOTTOM-WIND* has its name from being supposed formerly by the country people to arise from the bottom of those lakes which are situated amongst mountains, for I know of none in a level country troubled with it. It is indeed puzzling enough to conceive why in a day when hardly a single breeze is a-stir, the surface of a lake, which is as smooth as a mirror, should without any apparent cause begin to be in motion, which in less than an hour rises to a considerable swell, with a direction sometimes to one quarter and sometimes to another ; yet such is really the case ; and similar appearances have been observed in some of the Alpine Lakes, though it has been imputed as a lie to Buchanan that he tells of a similar phenomenon being frequent in the lake of Lenox."
* This is extracted from the same author.
"The Bosom Wind—is quite a different affair, and takes place wherever one object in the direction of the wind overlooks another; or universally where any thing breaks the current of the air that would otherwise impinge directly on the objects beyond it. This is particularly the case where large rocks screen things below them from the direct force of the wind, yet subject them to what is called a Bosom Wind. Near the sources of the Caldew is a valley called Swineside, never visited by the rays of the sun during the winter months. On the northern side of the hill which overshadows it in this manner, and at a considerable height above the valley, is a pretty large bason of water, called Booth-scale Tarn, three fourths of which are surrounded by an exceedingly steep heath, or by entire rocks ; and the other fourth, being the side right above the valley, gives an outlet to the water. A road leads from the low grounds to the lake, and from the outlet winds about half round it, gradually ascending to some rocks where are slate quarries, on account of which it was first made; near these quarries the road is a considerable height above the lake, and the perpendicular heights of the hill above it can not be less than four hundred yards ; on the other side of this height the descent is at an angle of perhaps fifty degrees, but on this at a much greater. On a wet and windy day in Autumn I once took a ride with two companions to this lake ; the wind blew directly over the height which I mentioned, not striking upon us except in uncertain puffs, on account of the intervention of the bill; that is, the wind, impinging on the inclined plane of the other side of the hill, was compelled towards the summit of it in an oblique direction, its powers continually increasing, and itself being more and more condensed by the addition of fresh air pressing on its course in a similar diverted manner. This current at the summit met with the regular wind, and, after striking violently on the mass of air moving in higher regions, was, by means of a combination of the weight and motion of that air, at last repelled into the tranquil and stagnant air beneath, where there was not a resistance from motion, and thus occasioned the wind of which I am speaking. It was this wind, which amused me very much at that time; I was looking at the lake beneath, and saw it grow black near the centre; the spot, where this first appeared, changed directly into a livid appearance by being contrasted with the rest of the water, through which, from this spot, as a fixed point, rolled concentric circles of waves towards the circumference in a tumultuous manner, whilst the centre itself remained quite smooth and undisturbed.
The wind, which produced this agitation, immediately after ascended the sides of the bason, and affected us with a very great force ; I could also observe the heath on the other sides of the pool shook by the same, and in the same main direction from a centre, very forcibly. Such were the effects that I observed. I am told, however, that others have known a wind of the same kind in dry weather snatch the water out of the pool, and scatter it as spray through the whole of this imprisoned space." Kelds.-" There is an appearance on the surface of lakes, which we can not account for on any principle, either of optics or perspective. When there is no apparent cause in the sky, the water will sometimes appear dappled with large spots of shades. It is possible these patches may have connexion with the bottom of the laké, as naturalists suppose the shining parts of the sea are occasioned by the spawn of fish; but this is more probable that in some way they are connected with the sky, as they are generally in the country esteemed to be a weather-gage. The people will often say, it will be no hay-day to-day, the lake is full of shades! I myself never saw this appearance, or I might be able to give a better account of it; but I have heard it so often taken notice of, that I suppose there is at least some ground for the observation. Though after all, I think it probable these shades may be owing only to floating clouds. I have often, says Mr. Locke, remarked this appearance on the lake of Geneva, without being able to assign a satisfactory reason; and the people of the country, I mean the philosophic part of them, are equally at a loss. If the spots were the shadow of a passing cloud, a vapour dense enough to interrupt the rays of the sun would certainly when suspended in a clear sky be visible, and immediately account for the appearance. But perhaps the effect may be derived from a cause diametrically opposite to the density of vapour. Let us suppose a partial rareness of the vapours dissolved in the atmosphere just above the spot, while every other part of the sky sheds light, by the reverberation of rays on the surface of the lake, that part sheds but little, and leaves a corresponding spot on the water, which compared with the splendour of the surrounding parts appears dark. The state of the sky may very well be considered as a weather-gage, because partial rarefactions destroy the equilibrium of the air.