The pressure of the superincumbent column in a great measure causes the density of the atmosphere, and therefore decreases in proportion to the height as the pressure of the column constantly de creases, yet the density in the torrid zone does not decrease so rapidly as in the temperate and frigid, as the column is longer, and because there is a larger pro portion of air in the upper part of it. This fact agrees with the assertion of M. Cas san, "that the barometer only sinks half as much for every two hundred feet of elevation in the torrid, as in the temperate zones." The density at the equator, though less at the surface of the earth, must equal at a certain height, and still higher exceed the density in the temper ate zones, and at the poles.
It was ascertained that a current of air constantly ascends at the equator, part at least of which reaches to and remains in the highest part of the atmosphere ; the fluidity of that body prevents it from tie cumulating above the equator, and hence it must descend the inclined plane before mentioned. The surface of the atmos phere being more inclined in the north ern hemisphere during our winter than that of the southern, more of the current must flow on the northern than on the southern, from which Cause the quantity of our atmosphere is greater in winter than that of the southern hemisphere ; in the summer it is just the contrary; cense quently, the range of the barometer is less inaummer than nn winter, and the greatest mercurial heights occur during winter. The heat of any given place in a great measure influences the density of its at mosphere ; that density will be most con siderable where it is coldest, and its co lumns shortest. Chains of mountains, the summits of which are covered with snow great part of the year, and highlands, must be colder than places less elevated in the same latitude, and the column of air over them much shorter. The current of air above must be impeded and accu mulate while on its passage over these places towards the poles, which causing an agitation, it will be communicated to, and indicated by, the barometer in a sin gular manner. These accumulations oc cur over the north-west parts of Asia and North America, and this raises the baro meter, and causes less variation in it there than in Europe. it is precisely so on the Pyrenees, the Alps, and the mountains in Africa, Turkey in Europe, Tanury, and Tibet. After the accumulations have ex
isted some time, the surrounding atmos phere becomes incapable of balancing the density of the air, when it descends with violence, and occasionscold winds, which raise the barometer; it is to this that we are to attribute the rise of the barometer almost always attending north-east winds in Europe, which is the effect of accumu lations near the pole, or in the north-west parts of Asia; it is thus, besides, that the north-west wind from the mountains of Tibet raises the barometer at Calcutta. It may be supposed that in the polar re gions large quantities of air are casually compressed ; when this is the case, the southern atmosphere must rush in to re place it, which occasions south-west gales and the fall of the barometer.
The mean heat of our hemisphere vary ing in successive years, the density of the atmosphere, and necessarily the quantity of equatorial air passing towards the poles, cannot be otherwise than variable; hence occurs the different ranges of the barometer in successive years ; at some particular periods, more considerable ac cumulations take place in the highest parts of Asia, and the south of Europe, than at others, which may be produced by early falls of snow, or the interruption of the sun's rays by long continued fogs; at such times the atmosphere in the polar regions becomes proportionably lighter, and this causes the prevalence of south erly winds in some winters more than in others. The heat of the torrid zone never greatly varying, the height and density of the atmosphere undergoes but fe w chang es, thence arises the comparatively small range of the barometer within the tropics, which gradually increases towards the poles as the difference of the tempera ture, and the density of the atmosphere increases with the latitude. The sinking of the barometer preceding violent tem pests, and the oscillations during their continuance, prove that very great rare. factions, or even destruction of air, in some part of the atmosphere, produces these phenomena; the fall too that accom panies winds arises from the same cause. Unfortunately we are but little acquaint ed with the operations which produce rain, consequently we are unable to ex plain satisfactorily why the barometer falls immediately preceding it.