94* Experimental Philosophy. [Lecture 7. 



than water. The vapours, therefore, can only 

 rise to a limited height; and it is generally 

 agreed that there are no clouds at the height of 

 four or five miles in the atmosphere : their usual 

 height, indeed, seldom exceeds a mile, nor very 

 often half a mile. Vapour, by coming in contact 

 with a cold body, can be deprived of its heat, and 

 is suddenly condensed into water again, as in the 

 refrigeratory of a still, where the vapour, confined 

 in a spiral tube, is made to pass through cold 

 water, and is condensed, as in the steam engine, 

 which was noticed in a former lecture. 



If, therefore, "the vapours in the atmosphere, 

 by ascending into the colder regions of the air, 

 by electricity, or by meeting with cold winds, 

 are deprived of the heat which keeps them in 

 the vaporific state, they will of course be con- 

 densed to clouds, and will fall down in the form 

 of ram. Perhaps the attraction of the earth, 

 when they approach it, may, in many cases, serve 

 to draw off the superfluous heat, or electricity, and 

 condense the vapours; which may account for its 

 generally raining on the tops of mountains, and 

 for the changes of the weather predicted by the 

 barometer. For when the air is so far rarefied as 

 not to be able to support the column of mer- 

 cury to a certain height in the tube of the ba- 

 rometer, it is generally regarded as a sure pro- 

 gnostic of rain. 



The air in the higher regions being sometimes 



