THE WEATHER. 171 



portion of it has some weight ; and softly though it 

 moves, still it has some friction. Both of these offer 

 some resistance to the cause of heat, and thus the 

 air is always a little warmed before it begins to 

 move in obedience to the heat. The resistance is 

 the greatest where the pressure is greatest* and the 

 air in consequence the most dense ; and that is one 

 of the reasons why the air is hotter in hollows than 

 on heights. If one were to ascend till the air had 

 only half the pressure, and consequently only half 

 the" density and the resistance to friction that it has 

 at the mean level of the sea, then it would yield twice 

 as easily to the heating cause ; and the same cause 

 that would render it not only warm, but disagreeably 

 hot at the level of the sea, would not bring it per- 

 haps even to the heat at which ice melts at the great 

 elevations. At less difference of height, the differ- 

 ence of resistance would be less than that ; but there 

 is a difference even for the smallest difference of 

 height, although our observation will not reach the 

 very minute cases, any more than it will reach the 

 very minutest ends in any department of observa^ 

 tion. We see as much, however, as may suffice to 

 convince us that the law is general ; and that is all 

 which is required for the purposes of knowledge. 



The daily and annual motions of the earth (and 

 the atmosphere moves along with it) cause the heat 

 of the sun to fall differently on every place, on every 

 day in the year, and at every hour of the day ; and 

 the different effects of the heat of the sun on differ- 

 ent kinds and forms of surface further increase the 

 variety, till the relative portions of heat, at any con- 

 siderable number of places, for any one time, cannot 

 be calculated with any thing like certainty, or indeed 

 at all. 



If that could be done, we should all be perfectly 

 " weather-wise," and should be able to tell how the 

 sky appeared and felt in distant places, and how it 

 would appear and feel at future times, with just as 



