30 THE YEAR. 



lestial body or motion much influence there, unless 

 through the medium of what may be termed ter- 

 restrial agents ; but it has some direct effect upon 

 those agents, and some secondary effect upon the 

 solid earth, or at all events upon the surface, the 

 question in which we are chiefly interested, and to 

 which we must in a great measure confine our na- 

 tural history. 



The terrestrial agents alluded to are the atmos- 

 pheric air and the water of the ocean; the former 

 of which every where surrounds the earth, to the 

 height of about fifty miles above the mean surface, 

 or forty-five miles above the very highest mountain ; 

 and the latter covers a great part of the surface. Those 

 two fluids have less extension and weight than the 

 solid parts of the earth, these being bulk for bulk 

 more than four and a half times the weight of water, 

 and not much less than four thousand times the 

 weight of air, at the mean level of the earth's sur- 

 face, and about that degree of heat which we call 

 temperate. But the air has such a tendency to ex- 

 pand when not restrained by pressure, that it be- 

 comes wonderfully rare or thin in the upper parts 

 of the atmosphere, so that the whole 50 miles in 

 height is only equal to about 30 feet of water ; 

 and consequently the whole mass of it, taking bulk 

 for bulk, is only a very small fraction of that of the 

 earth. The air is perfectly restless, too, and with 

 the least difference of heat or pressure it is instantly 

 set in motion. When it is put powerfully in motion, 

 we know what effects it can produce, not only upon 



