PREFACE. 



IT is now about thirteen years since I became acquainted 

 with Dalton's experiments on the aqueous vapor which ex- 

 ists in the atmosphere. I was much struck with one of his 

 results; namely, that the quantity of vapor in weight, ex- 

 isting at any time in a given space, could be determined 

 with great accuracy in a few minutes, by means of a ther- 

 mometer and a tumbler of water cold enough to condense 

 on its outside a portion of the vapor in the air. It occurred 

 to me at once, that this was the lever with which the mete- 

 orologist was to move the world. 



I immediately commenced the study and examination of 

 atmospheric phenomena, determined to discover, if possible, 

 what connection there is between rain and the quantity of 

 vapor, in the atmosphere ; but the more facts I collected, 

 whether from my own observations or from those of others, 

 the more contradictory and perplexing they became. It 

 had been long known that vapor is lighter than air, and it 

 was inferred from this, that when a portion of atmospheric 

 vapor is condensed into cloud, the air in the cloud becomes 

 specifically heavier than it was before. This doctrine I re- 

 ceived as an axiom, and I never for a moment doubted it, 

 until it occurred to me to calculate the effect which the evo- 

 lution of the latent caloric produces, during the formation of 

 the cloud. 



^?l -f^ 'S. ' H- 



