APPLICABLE TO MATHEMATICS, &C. 337 



which take place in the weight of the barome- 

 trical pressure in the different latitudes of the 

 earth's surface, has fully and satisfactorily ex- 

 plained the circumstance of the barometer rising 

 when the rain is falling, and falling when evapo- 

 ration is going on. He states that, " it must be 

 allowed that water, when changed into vapour, 

 constitutes a part of the atmosphere, for the time, 

 and weighs with it accordingly ; also, that when 

 vapour is precipitated in form of rain, the atmos- 

 phere loses the weight of it. But it would be 

 too hasty to conclude from hence, that where 

 evaporation is going forward the barometer must 

 rise, and where rain is falling it must fall also ; 

 because air loaden with vapour is found to be 

 specifically lighter than without it. Evaporation, 

 therefore, increases the bulk and weight of the 

 atmosphere at large, though it will not increase 

 the weight over any particular country ; if it 

 displace an equal bulk of air specifically heavier 

 than the vapour, and in like manner, rain at any 

 place may not diminish the weights of the air 

 there, because the place of the vapour may be 

 occupied by a portion of air specifically heavier." 

 — (See Dr. Dalton's Meteorological Essays, p. 

 96 and 97.) 



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