294 WRITINGS OF JOSEPH HENRY. [1855- 



in subordinate particulars modifications will be required, 

 yet we think the general propositions of his theory will stand 

 the test of time. 



As a general rule previous to the commencement of an 

 extended storm (during winter), the surface current is from 

 the southwest or some southerly direction, the temperature 

 rises and the pressure of the air diminishes as indicated by 

 the fall of the barometer. This state may continue for sev- 

 eral days, and we think it is produced by the southerly cur- 

 rent increasing in quantity, in velocity, and depth, thereby 

 rendering the stratum of air next to the surface of the earth 

 abnormally warm and moist, and consequently lighter, while 

 the upper current remaining the same, the atmosphere above 

 the surface of the earth gradually assumes a state of tottering 

 equilibrium. This condition, according to Mr. Espy, is not 

 brought about by the gradual diminution of the density of 

 the lower stratum but by the increased density of the upper 

 strata, due to the radiation into space of the latent heat 

 which had been evolved during a previous storm. We think 

 however that both causes are operative. This instability or 

 tottering equilibrium will first take place at the far west, on 

 the western plains east of the Rocky Mountains, since (as we 

 have before said) the commotions on the western side can be 

 but slowly propagated across the high mountain system. A 

 storm then consists of the ascent of the lower current into 

 the upper and the gradual transfer of the commotion of the 

 air eastward. To take the simplest case, let us suppose the 

 storm to be of circumscribed character, like that of a water 

 spout or thunder storm. In this case after the unstable 

 equilibrium has been produced, the slightest disturbance, 

 such as the passage of the lower current over a slight eleva- 

 tion or over ground more highly heated than the adjoining 

 will tend to establish an upward current. The light, warm 

 and moist air below will be buoyed up with great rapidity 

 and as it ascends will come under less pressure and will ex- 

 pand into a larger bulk. If it were perfectly dry it would 

 again be in equilibrium, its bulk would be increased, its 

 density would be diminished to that of the air to which it 



