6 



ON THE SEPABATE PBESSURES OF THE AQUEOUS AND 



is converted into particles of water which for some time float 

 in the air. Now these particles, being sustained by the gases, 

 their weight is transmitted downwards through the gases, and 

 it must increase the total gaseous pressure on the surface. 

 As the sun rises in the heavens, temperature and evaporation 

 increase, and more aqueous matter passes into the atmosphere, 

 to remain for a time vapour, and then to become water in the 

 upper regions; and, the process continuing whilst the tempera- 

 ture is increasing, the total weight of the aqueous matter, in 

 both the aeriform and the liquid state, is increased. 



That this process really takes place, particularly in warm 

 latitudes, is sufficiently evident from the daily evaporation 

 that is known to be taking place from all wet surfaces. With 

 the daily rise of temperature a certain weight of exposed 

 water disappears in a given time, and it is known to pass 

 into the atmospheric space above, where it can be followed 

 and detected: and it must accumulate there in some way 

 until it is formed into rain or snow. But until such rain 

 or snow fall, the aqueous matter, having a certain weight, 

 must remain in the atmosphere in one or both of the two 

 forms described, and that weight must constitute a part of 

 the general atmospheric pressure. 



In latitudes distant from the equator, the temperature over 

 the sea is generally lower in proportion to that distance, 

 and the daily increase of evaporation is there less palpable 

 than near the equator, but results similar in their nature will 

 always be found, as fresh vapour to some extent is every day 

 sent into the atmosphere in all latitudes, and some of it is 

 ordinarily condensed in upper regions, and becomes either 

 incipient or palpable cloud floating in and resting upon the 

 air, thus increasing the total atmospheric pressure. And, 

 provided that no other alterations were taking place at the 

 time, careful observation of the barometer might no doubt 

 detect the daily change, and shew it to be the greatest in 

 the hottest and the least in the coldest latitudes. The whole 



