-1859] WRITINGS OP JOSEPH HENRY. 295 



had ascended, and its temperature would be the same as that 

 of the surrounding stratum. But since it contains moisture 

 and in expanding becomes colder, a portion of the vapor will 

 be condensed, and in this condensation will give out its latent 

 heat. Hence the air of the column will be warmer than 

 that of the surrounding atmosphere; it will consequently rise 

 to a greater height, again expand, again become colder; 

 another portion of vapor will be condensed, and another 

 amount of latent heat evolved, and thus the air will rush 

 up with an accelerated velocity, and probably gather mo- 

 mentum sufficient to carry it to a height greater than that 

 due to its buoyancy alone. The condensed vapor will fall in 

 rain through the base of the cloud, the air on either side of 

 the storm will be forced out from the uprising column into 

 the surrounding air, and while the pressure at the base of 

 the column will be diminished, that on each side will be in- 

 creased, hence the barometer will be frequently found to rise 

 slightly before the approach of a storm and to sink rapidly 

 as the centre of the uprising column approaches the place of 

 observation. 



A series of observations has been made at the Smithsonian 

 Institution to determine the variations of the barometer 

 during the passage of thunder storms, and in every case in 

 which observations of this kind have been obtained, a sudden 

 fall has been observed in the barometer, and at the moment 

 of the descent of the rain a slight elevation, followed again 

 by a depression and then a rise, until the normal pressure 

 of the day, or perhaps a little greater, has been obtained. 

 The intermediate rise taking place at the moment of the fall 

 of the rain may be properly attributed to the momentum of 

 the drops as a sufficient cause. 



Fig. 10 is intended to illustrate the conditions and phenom- 

 ena of a commotion of this kind. The dotted space, c d, at the 

 bottom represents the lighter atmosphere, consisting of the 

 warm southwest current sur-charged with moisture; above 

 this the parallel horizontal lines, a b, and the arrows, indicate 

 the direction and position of the upper western current. The 

 ascending column is represented by the upward turned arrows 



