METEOROLOGY, is the science of studying the phenomena of the atmo sphere, and the term by which is express ed all the observations that tend to make them a system. There are many most im portant meteorological phenomena, and those may be classed under five distinct beads; for instance, the alterations that occur in the weight of the atmosphere, those that take place in its temperature, [he changes produced in its quantity by evaporation and rain, the excessive agita tion to which it is frequently subject, and he phenomena arisingfrom electric and other causes, that at particular times oc ::asion or attend the precipitations and agitations alluded to.
All the above phenomena prove to de nonstration, that constant changes take dace, the consequences of new combina ions and decompositions rapidly follow ng each other. The majority of mete orological alterations depend on these chemical changes, and were we accu rately acquainted with the peculiarities of all the substances which form the component parts of the atmosphere, no thing would be more easy than to explain the result of their mutual action ; but as that is unfortunately not the case, we must be contented to build upon strong probabilities, supported in many instances by positive experiment.
It is singular, that this science should have remained for so long a period in a state of comparative neglect, when it is recollected, that almost all the operations necessary for the support of human life, and almost all the comforts of corporeal feeling, depend upon the state of the at mosphere, and yet nothing was attempted to any purpose towards investigating the laws of meteorology till the seventeenth century, when the most important dis coveries of the barometer and thermo meter occurred, which was followed in the eighteenth by the invention of ex cellent hygrometers and electrometers ; by these the philosopher finds himself competent to make accurate and satis factory observations. Scientific persons, who have particularly turned their atten tion to this pursuit, have undertaken the laborious task of colle:ting and metho dically arranging numbers of the obser vations just mentioned, and after atten tively comparing and examining them, have formed theories of the weather of more or less probable accuracy ; but the science is of such difficulty, that though those theories deserve every praise, we are compelled to acknowledge the pheno mena of the weather is still very imper fectly understood. This acknowledge ment, however, reflects no discredit on those ingenious men, as it is impossible that any thing like certainty should be attained, till observations that can bt de pended upon are procured from all parts of the globe, the atmosphere has been more accurately explored, and the Che mical changes occurring in it are correct ly ascertained.
To render our explanation of this sub ject as satisfactory as circumstances will permit, we shall proceed in the success lion before pointed out ; with respect to the changes in the weight of the atmos phere, it is generally known that the in strument called the barometer shews the weight of a body of air immediately above it, extending to the extreme boundary of the atmosphere, and the base of which is equal to that of the mercury contained within it. As the level of the sea is the
lowest point of obsersation, the column of air over a barometer placed at that level is the longest to be obtained ; in this case the mean height of the barometer is thirty inches. According to the experi ments of Sir George Shuckburgh in the Channel and the Mediterranean Sea, in the temperature of .55° and 60°, this was found to be the case, and the result is confirmed by those of M. Ilouguer on the coast of Peru, in the temperature of 84°, and Lord Mulgrave in latitude 80°. From these data it is evident, that the mean height of the barometer decreases in pro portion with its elevation above the level of the sea, and in proportion to the con sequent shortening of the columns of air; hence it is used for measuring heights. The keeping of a barometer in one par ticular place does not make the mercury stationary, as it will vary by rising or falling to the extent of several inches; of necessity the weight of the air which balances the mercury must be subject to the same changes; this circumstance proves that the gravity of the air in any given situation varies greatly, being at one time light, and another heavy, an effect which must be caused by changes in its quantity, and a fact that demon strates the air of every place liable to perpetual alterations, which must arise from the accumulation of air in particular places, and a reduction in others, " or,* as Dr. Thomson observes, " part of the atmosphere must be alternately abstract ed altogether, and restored again by some constant, though apparently irregular process." The variations of the barometer be tween the tropics are very trifling, and it is worthy of observation, it does not de scend more than half as much in that part of the globe for every two hundred feet of elevation as it does beyond the tropics, which we learn from the Journal de Physique ; besides, the barometer rises about two-thirds of a line twice during each day in the torrid zone. We are in formed by M. Horsburgh, that from lati tude 26° north to latitude 27° south, which includes the space termed the tropical seas, the mercury attained its greatest elevation at eight in the morn ing, flom which hour till noon it continu ed stationary ; it then began to fall, and descended till about four o'clock, when it reached the lowest point of depression. In the interval between four and five the mercury rose, and continued to rise till about nine or ten P. M. when it had once more arrived at its most elevated point,.