200 PIONEERS OF SCIENCE IN AMERICA. 



in weight, existing at any time in a given place, could be deter- 

 mined by means of a thermometer and a tumbler of water cold 

 enough to condense on its outs.ide a portion of the vapour in 

 the air. "It occurred to me at once," Prof. Espy says, "that 

 this was the lever with which the meteorologist was to move 

 the world. I immediately commenced the study and exami- 

 nation of atmospheric phenomena, determined to discover, if 

 possible, what connection there is between rain and the quan- 

 tity of vapour in the atmosphere." Prof. Espy prefaced his 

 paper in the British Association by saying that he had found, 

 by examining simultaneous observations in the middle of storms 

 and all round their borders, that the wind blows inward on all 

 sides of a storm toward its central parts ; toward a point if the 

 storm is round, and toward a line if the storm is oblong, ex- 

 tending through its longest diameter. The theory is, in brief, 

 that every atmospheric disturbance begins with the ascension 

 of air that has been rarefied by heat. The rising mass dilates, 

 and, as its temperature falls, precipitates vapour in the form 

 of clouds. Owing to the liberation of the latent heat, the dila- 

 tation continues with the rising till the moisture of the air form- 

 ing the upward current is practically exhausted. The heavier 

 air flows in beneath, and, finding a diminished pressure above 

 it, rushes upward with constantly increasing violence. The 

 great quantity of aqueous vapour precipitated during this 

 atmospheric disturbance gives rise to heavy rains. Much of 

 this theory still holds good ; but it has been found that the 

 motion of the wind in storms is rotary. 



Besides his explanation and proofs of this theory, Prof. 

 Espy presented to the British Association a paper on Four 

 Fluctuations of the Barometer. The theory was more fully 

 elaborated in The Philosophy of Storms, which was published 

 in a large octavo volume by Little, Brown & Co., Boston, in 

 1841, and was re-enforced by detailed descriptions of a large 

 number of storms occurring on the land and the ocean, the 

 course of which the author had been able to follow and study 

 with considerable accuracy. It also contained his answers to 

 the criticisms which had been made against his theory in the 

 British Association and elsewhere by prominent men of science 

 and rival meteorologists. In it, furthermore, he defended his 

 theory that storms could be produced by large fires making 

 local disturbances in the equilibrium of temperature, whence 



