THE GASE0U8 POUTlONS OF THE ATM08PHEEE. O 



that hangs over the tropical seas, which mist is made up of 

 minute particles of water that have been produced by cold 

 from the ascending vapour, and this mist is floating in the 

 mass of the gases. These particles of water, however, though 

 each may be small, are assuredly not destitute of weight, and 

 whatever weight they may have must be added to that of the 

 body in which they are floating, just as the weight of wood, 

 or any other light substance that may be put into water con- 

 tained within a vessel, will be added to the weight of the 

 water that is in it, and will increase the total pressure on the 

 bottom of the vessel. And the weight of the particles of 

 water that are thus floating in the atmosphere, though sus- 

 tained immediately by the gases, is added to their weight, and 

 the weight of the liquid particles finally increases the density 

 of the gases near the surface, and augments their pressure 

 upon it. The total atmospheric pressure then, — that which 

 is measured by the barometer, is, under such circumstances 

 as have been described, made up of three, and not of two 

 parts : first, that of the aeriform substance, aqueous vapour, — 

 secondly, that of the atmospheric gases, — and thirdly, that 

 of the particles of water that are floating in, and are princi- 

 pally sustained by, the gases ; and of these three portions the 

 first and the last are frequently undergoing changes which 

 considerably vary their quantities, and consequently their 

 weight and pressure on the surface of the earth and on the 

 barometer. 



Over the sea, near the equator, where the dew-point is at 

 80°, the temperature during the night will be generally about 

 the same as the dew-point, and therefore little or no evapora- 

 tion will take place at night. But the rising sun increases 

 the temperature, when evaporation begins, and additional 

 vapour is generally thrown into the atmosphere, increasing 

 its total weight and pressure. But as this vapour expands 

 successively into the upper regions, some of it is condensed 

 by the cold of the gases through which it has to pass, and 



