182 ' NINTH ANNUAL REPORT OF 



To comprehend the nature of the changes which the atmosphere un- 

 dergoes, the elementary principles of meteorological science must he 

 understood; but I do not intend to enter more minutely into these than 

 will enable those who have not devoted much attention to the subject 

 to comprehend the more important truths. 



The nature of the atmosphere was long involved in obscurity. Its 

 properties could not be ascertained till chemistr}' and other branches of 

 natural science were considerabty advanced. Air has so little color 

 that it is almost invisible, and offers so little resistance to motion, that 

 it was considered by the school of Aristotle imponderable. This 

 opinion was entertained for many centuries afterwards, until the inven- 

 tion of the barometer, by Toricelli, in 1640, and the discovery of the 

 fact pointed out by Pascal, that the barometer stands lower on the top 

 of a mountain than at its base, left no doubt remaining that air was pos- 

 sessed of weight, and, consequently, that the atmosphere exerted a 

 great pressure. 



If a glass tube, three feet in length, be filled v/ith mercury, and its rt-pen 

 end inverted in a basin of the same liquid, the mercury in the tube will 

 stand, at the level of the sea, nearly 30 inches higher than the surface 

 of that in the basin. This column of mercury, which, if its section is a 

 square incli, weighs nearly 15 pounds, is balanced by a column of air 

 of the same section and extending to the top of the atmosphere. The 

 pressure of the atmosphere is, therefore, equal to that of an ocean of 

 mercury of 30 inches deep, or to a pressure on each square inch of sur- 

 face of about 15 pounds. Moreover, mercury is 13^ times heavier than 

 water, and 10,500 times heavier than dry air at the surface of the 

 earth; hence the pressure of the atmosphere is equal to that of an 

 ocean of water of about 33 feet deep, or an ocean of air, of equal density 

 throughout, of 27,000 feet high. 



That the atmosphere should press on the surface of the earth, and on 

 all parts of our bodies, with a weight of 15 pounds to the square inch, 

 is, at first sight, a very perplexing tijct; but it is fully illustrated by the 

 familiar and analogous pressure of water. The diver who descends 

 below the surface of the sea is pressed on all sides by the superincum- 

 bent weight of water, and, instead of being incommoded by this, is ren- 

 dered more buoyant. The particles of the air are of extreme tenuity 

 and of almost perfect mobility, and therefore offer no resistance to 

 bodies moving among them. On these accounts the weight of the air, 

 and the great pressure of the atmosphere, remained so long concealed. 



For a long time after the discovery of tlie pressure of the atmosphere, 

 the world remained in ignorance of its chemical and mechanical con- 

 stitution. The analysis'of air was, however, one of the first trmmphs 

 of modern chemistry. Rutherford discovered hydrogen, one of its com- 

 ponents, in 1772; and, two years after. Priestly and Shiel, independ- 

 ently of each other, discovered the other principal ingredient, namely, 

 oxygen. 



Dry air is composed of 77 parts nitrogen, 23 oxygen, by weight; 79 

 parts nitrogen, 21 oxygen, by volume; carbonic acid, 1.1000 by 

 weight; ammonia, only a trace. The atmospliere also contains a cer- 

 tain amount of moisture. This is the only component which is liable to 



