1369. The variations in the temperature of the air in any particular place, exclusive of the differences of seasons and climates, are very considerable. These changes cannot be produced by heat derived from the sun, as its rays concentrated have no kind of effect on air; these, however, heat the surface of our globe, from which heat is communicated to the immediate atmosphere; it is through this fact that the temperature is highest where the place is so situated as to receive with most effect the rays of the sun, and that it varies in each region with the season; it is also the cause why it decreases in proportion to the height of the air above the surface of the earth. The most perpendicular rays falling on the globe at the equator, there its heat is the greatest, and that heat decreases gradually to the poles; of course the temperature of the air is in exact unison: from this it appears that the air acquires the greatest degree of warmth at the equator, whence it becomes insensibly cooler till we arrive at the poles; in the same manner the air immediately above the equator cools gradually. Though the temperature sinks as it approaches the pole, and is highest at the equator, yet as it varies continually with the seasons, it is impossible to form an accurate idea of the progression without forming mean temperature for a year, from that of the temperature of every degree of latitude for every day of the year, which may be accomplished by adding together the whole of the observations and dividing by their number, when the quotient will be the mean temperature for the year. The 'diminution,' says Dr. Thomson, from the pole to the equator takes place in arithmetical progression; or, to speak more properly, the annual temperatures of all the latitudes are arithmetical means between the mean annual temperature of the equator and the pole; and, as far as heat depends on the action of solar rays, that of each month is as the mean altitude of the sun, or rather as the sine of the sun's altitude. Later observations, however, have shown that all the formulﾵ for calculating the mean temperatures of different latitudes, which are founded on Mayer's Empirical Equation, though tolerably accurate in the Northern Atlantic Ocean, to; latitude 60ﾦ, are totally irreconcilable with observations in very high latitudes) and on the meridians, from 70ﾦ to 90ﾦ W. and E. of London. The results of late arctic voyages, and of Russian travels, have been satisfactorily shown by Sir David Brewster (Edin. Phil. Tr.), to prove the existence of two meridians of greatest cold in the northern hemisphere; and the mean temperature of particular countries varies, not only according to the parallels of latitude, but also according to their proximity to these two cold meridians. (T.)
