250 ASTRONOMY. 



to circumstances, for it loses nothing of its substance, nor transform* 

 into another element. This peculiarity also excludes the possibility of 

 analysis, or divisions of its parts. The elasticity of the air which gives 

 it transparency, is that quality which enables us to see objects distinctly; 

 the light passes through it to the organ of sight, and pene- 

 trating to the optic nerve, pictures the form of substances, by reflection, 

 upon the visual tablets. Again, the atmosphere is most admirably 

 adapted to sustain our existence, by the power it possesses of purification, 

 and the exact adaptation of its substance for animal respiration; were it 

 more dense than it is, we should be in continual darkness ; were it more 

 thin and rarified, we could not breathe in it, but must soon expire. 

 The atmospheric air is composed of two gases, oxygen and nitrogen, 

 with a very small proportion of carbonic acid, and water in a state of 

 vapour. The two last are considered as accidental ingredients, and not 

 constituent parts, as well as on account of the smallness of their quantity, 

 as because they occur in different proportions at different times. 



THEORY OF WINDS. 



Heat is the principal cause of winds. It must be evident, that as the 

 rays of the sun descend perpendicularly on the earth under the torrid 

 zone, that in those regions a much greater quantity of heat must be 

 communicated than in the more oblique countries towards the Poles. 

 The heat thus acquired, rarifies the air, and causing it to ascend; the 

 vacuum which follows is immediately filled from the north and south, 

 which being of a cold nature, the fierce heats of the equatorial regions 

 are so modified as to become bearable. Thus two winds, north and 

 south, would be generated ; but these would be afterwards modified and 

 changed. For example; the diurnal motion of the earth gradually 

 lessens to the Poles from the equator, where the motion is communicated 

 to the atmosphere in an equal degree ; it is evident that if part of it was 

 conveyed suddenly from a temperate latitude, it would not directly 

 acquire the velocity of that at the equator, consequently the earth would 

 outstrip it in speed, and as she moves from west to east, the mountainous 

 ridges would strike against it, and, driving it forward, an east wind would 

 be the result. Land and sea breezes, trade winds, regular and variable 

 winds, are all accountable for on the above principle, modified, however, 

 by various other influences, such as the motions of the sea under the 

 guidance of the moon, chemical changes in the elementary constituents 

 of the atmosphere, Sec. &c. The prevailing winds in this country, as 



