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Sect. i. Explanation of the principle of Atmospheric Decep-
NATURE AND ART
N A T U R E.
We now advance to the atmospherical phænomena of the science of Geology, in the extensive sense in which we have em. ployed this term; and shall proceed to a brief survey of whatever is most curious or worthy of observation in the composition of the Atmosphere; the variation of Climate; the extremes of Heat and Cold; the existence of Electricity and electric Powers, as Thunder and Lightning, Aurora Borealis, Water-spouts ; Falling-stars, and Ignes Fatui ; Echoes, Wind, Hurricanes, and Storms; the origin of Mist, Dew, Sleet, Snow, Mirages, or Fata Morgana, Meteoric, Stones, and various other appearances connected with or depen. dent apon the preceding; and which collectively constitute the Science of Meteorology.
GENERAL NATURE OP the ATMOSPHERÉ.
THE atmosphere is that invisible elastic Auid which surrounds the earth to an unknown height, and incloses it on all sides. It was thus denominated by the Greeks in consequence of the vavours which are continually mixing with it, or combined in it *. Iu contemplating the nature of the atmosphere there are two points of considerable importance to be attended to, respecting which therefore we shall offer a summary of the best established facts and opinions of the present day; and these are the materials that enter into its composition, and the changes to which it is liable.
Composition of the Atmosphere. Neither the properties nor the composition of the atmosphere seems to have occupied much of the attention of the ancients. Aristotle considered it as one of the four elements, situated be. tween the regions of water and fire, and mingled with two exhalations, the dry and moist; the first of which occasioned thunder, lightning, and wind; while the second produced rain, snow, and hail. The ancients, in general, seem to have considered the blue colours of the sky as essential to the atmosphere; and several of their philosophers believed that it was the constituent principle of other bodies, or at least that air and other bodies are mutually conyertible into each other. Thus Lucretius :
Semper enim quodcunque fuit de rebus, id omno
# From årrus, a vapour, and paipa, a sphere,