the mean heat of the two (after the combination) can 

 contain, the excess is precipitated : 



in* 



Let temp, of one stratum =65 its tension or elasticity =0*61 by table. 

 Let temp, of the other... =41 ... ... 0-27 



2|106 0-88 



Mean temperature after "I , o Mean tension after \ <Q.AA 

 combination J combination J ' 



By table, the tension of vapour at 53 is 0'40 inch. 



Therefore vapour of the tension of 0'04 inch of the 

 mercurial column is precipitated in cloud, fog or rain. 



Heat and moisture are the principal causes of the 

 variations in the weight of the atmosphere, and neces- 

 sarily of the variations in the barometer ; the moon is 

 considered to have some influence ; but if she exert any 

 power in causing accumulations or tides in the atmo- 

 sphere, her action on the barometer, computed to be 

 about y^ of an inch, is so small, that even with the 

 most delicate instruments and the most accurate ob- 

 servers we can scarcely hope to demonstrate it satis- 

 factorily. 



The variations of the barometer are less within the 

 tropics than in the temperate and polar regions ; they 

 vary in different countries in the same latitude, and they 

 are great in mountainous countries and islands ; in Peru 

 the range of the mercury is about ^ of an inch, in Lon- 

 don 2~ inches, and in St. Petersburg it exceeds 3 inches. 



The pressure of the atmosphere at the level of the 

 sea, the barometer at 30 inches, is 15 Ibs. on the square 



