Our limits will not permit us to enter into a minute detail of the maxims alluded to above, nor are they of so much importance as to require a particular enume ration. It may be useful, however, to notice some of the most popular, and what have been generally con• sidered the most certain prognostications of approach ing changes in the state of the weather, as serving at once to illustrate the history of meteorology, and to show what are still the most important desiderata on this branch of physical science. It would be difficult, indeed, if not impossible, to make such a classification of these prognostics, as to include the various and of ten Fanciful opinions, that have prevailed on this sub ject. The most commonly received, however, may be arranged under the two following classes : 1st, The ap pearance of the sky ; and, 2d, Phenomena that take place at or near the surface of the earth. In illustrating these, we shall have an opportunity of noticing certain atmospherical phenomena which cannot well be omit ted, but which could not with so much propriety be in troduced in any other part of this article.
To the first class belong those luminous bodies that occasionally appear in the atmosphere. and which have been denominated meteors, or failing stars. These bo dies appear to be of different magnitudes, and even of various forms, though this last circumstance may per haps be the effect of optical deception. In general they seem to be globular, continuing visible only for a few seconds, and moving with great velocity. Their course is on some occasions in a straight line, and on others curvilineal, rendered more distinct by the tail or luminous train which they leave behind them ; and be fore disappearing, they are sometimes separated into se veral smaller bodies, accompanied with an explosion ;resembling thunder, more or less loud according to their magnitude or distance. It was long supposed, and has now been proved by the most incontrovertible evi dence, that these explosions are followed by a shower of solid bodies of a stony or metallic substa..ce, some of which have even appeared luminous in their descent after the explosion, and have been taken up before they had time to cool : (See NIETEORITE.) This last phe nomenon, indeed, is of comparatively rare occurrence. Thousands of small meteors, as various in magnitude and brilliancy as the fixed stars, have been seen in all seasons, and in almost every variety of weather, unaccom panied either with explosions, or the deposition of solid substances ; nor is it certain that even the larger and more luminous meteors, such as that of 1783, described by Cavallo, or one in 1811, an account of which was given by Professor Pictet in the Biblzotheque Britan rigue for lay 1811, are always followed by a fall of meteoric stones. On the other hand, these stones have sometimes been observed to tall after a loud detonation, when no meteor was visible, though this may perhaps be accounted for, from its having been obscured either by the superior light of the sun, or the intervention of clouds. But how( ver this may be, the appearance of large meteors, and the fall of meteoric stones, or, as they have very improperly been called, aeroliths, are phenomena that appear to be closely connected, and this is almost all that is known upon the subject. We deem it quite unnecessary, therefore, to enter into a minute account of the attempts that have been made to Glassily these luminous bodies, according to their form, colour, or magnitude. Whether they at e all ot the same origin, but varying in appearance, in consequence either of their different distances, or of some peculiar state of the at mosphere, or whether they are essentially different in their nature, are questions to which, in the present state of meteorological science, no answer can be given. As
prognostics of the weather, they have in general been supposed to predict wind, as appears from various pas sages in ancient authors ; and it is also commonly be lieved, that the wind which follows will blow from the point of the compass towards which the meteor is ob served to move. One at least of the various hypothe ses which have been proposed to account for these phe nomena is interesting, inasmuch as it appears to explain, in certain cases, the connection between the motion of the meteor and the direction of the wind.
The hypothesis to which we allude, is that which ascribes meteors to certain vapours arising from the earth, and becoming ignited in the higher regions of the atmosphere. The origin of this opinion may be traced to Aristotle ; but from the discoveries in che mistry, of which that author was in a great measure ignorant, it has assumed, in the hands of the modern philosopher, a more definite form. Halley, and after him De Luc, has endeavoured, on this principle, to account for some at least of the circumstances attend ing the appearance of luminous meteors. The latter supposes that falling stars proceed from a phosphoric fluid, ascending from some spot of the surface of the earth, which becomes visible only when, by decompo sition in the higher regions, it takes fire, and light is disengaged. If such a fluid can be supposed to rise in a continued column, without mixing with the at mosphere, or being dispersed by wind, there is no dif ficulty in conceiving how it may produce the appear ance of a falling star. When the upper extremity of the column has reached such a height as to be in a great measure above the region of the clouds and mois ture, it may, from the dryness of the air, take fire spon taneously, as phosphorus is known to do when expos ed to the atmosphere in its ordinary state ; and igni tion having once commenced, it may be communicated backward to successive portions of the column, till it arrives at a portion of the atmosphere sufficiently moist to extinguish it, or at some point where the column it self has been broken and separated. In these circum stances, it is obvious that the appearance would be precisely that of a 'ailing star ; and Mr. Forster has ingeniously applied the by pothesis, to account for the apparent relation between such phenomena and suc ceeding gales 01 wind. It has been long known that different, and even opposite currents of wind, may ex ist at different heights in the atmosphere at the same time ; and the author just referred to has found, from various experiments and observations, that when the wind near the surface of the earth changes, it frequent ly blows from the same point from which the current above had previously blown. He observes, therefore, that De Luc's hypothesis, though he is far from em bracing it as satisfactory, will sufficiently account for the relation above stated, by supposing that the column of phosphor ic fluid is bent, previous to ignition, in the direction of the upper current ; so that, when ignition commences, the luminous body moves towards the point from which that current then proceeds, and from which the lower current is afterwards to blow. It is a prognostication of wind, then, only in so far as it in dicates a change that has already commenced in the higher regions of the atmosphere, hut which has not yet taken place near the surface of the earth.