-1859] 



WRITINGS OP JOSEPH HENRY. 



303 



Fig. 11. 



Previous to the commencement of the storm there exists 

 over the surface of the United States a lower stratum of air 

 moving from southern points of the horizon, and over this 

 at an elevation of two or three miles the constant current 

 from the west continues its habitual and un-interrupted 

 course. The lower stratum, coming principally from the 

 Gulf of Mexico, is abnormally warm, moist, and light, while 

 the super-stratum is in its usual condition, and the whole is 

 therefore in a state of unstable equilibrium. At the far west 

 this lower stratum begins to be invaded by the denser air 

 from the polar current, which coming from the northwest 

 and mingling with the upper current, presses under, and 

 turning the moist current upward produces an ascending 

 column, or rather wall, which mingling with the upper cur- 

 rent is carried rapidly to the east. The upper current is 

 continued with varying energy, and by the condensation of 

 the vapor from the lower, forms clouds and rain, which are 

 carried in advance to the east, as the whole system of dis- 

 turbance is borne in the same direction by the ordinary 

 eastward flow of the upper current of the aerial ocean. The 

 primary lower current is shown in the figure by the stratum 

 a; the upper current, which has filled the whole space on 

 the west down to the earth, by bb; the portion of the prim- 

 ary upper current into which the stratum a has ascended 



