304 WRITINGS OF JOSEPH HENRY. [1855- 



aud in which its vapor has been condensed into clouds and 

 rain is represented by c. 



As the storm advances eastward, it leaves the country be- 

 hind it entirely covered with the westerly current, and in 

 this way carries before it to the ocean the greater portion of 

 the vapor with which the lower stratum, previous to the 

 commencement of the storm, was saturated. The rain which 

 falls at any given place is formed of the condensed moisture 

 which a few hours previous existed at the surface of the earth 

 in the same spot or its vicinity. 



We have represented in Fig. 11 the whole cloudiness 

 thrown eastward in advance of the storm, but in some cases, 

 with a more energetic upward motion, a part of the ascend- 

 ing air will be thrown out to the west above; but this can 

 scarcelj'' ever take place to the same extent as on the eastern 

 side. After the upward moving column has passed over a 

 given place, the wind which was previously from the east, will 

 suddenly change to the west, the sky will become clear and 

 a great reduction of temperature follow. The whole effect 

 then is due to the instable equilibrium produced in the air 

 by the introduction of moisture and the accompanying 

 elevation of temperature, together with the subsequent evo- 

 lution of the latent heat. A similar condition of the atmos- 

 phere preparatory to the formation of another storm will 

 gradually be re-produced. The westerly wind will again be 

 buoyed up by the warm air from the south, it will therefore 

 disappear at the surface of the earth, at which a calm will at 

 first exist, the southerly wind will increase in velocity, the 

 thermometer and hygrometer will indicate a higher tem- 

 perature and increasing amount of vapor, the barometer will 

 fall, and after a given interval another instable equilibrium 

 will be produced, to be followed by another subversion of 

 the strata of the aerial ocean and the repetition of all the 

 previous phenomena. The intervals between two successive 

 storms will also depend on the time of radiation into celes- 

 tial space of the evolved heat, in order to reduce the upper 

 stratum to its normal condition of temperature and density; 

 but the time required to produce these effects is frequently 



