302 WRITINGS OF JOSEPH HENRY. [1855- 



times several hundred miles across, and the cloudiness pro- 

 duced frequently overspreads simultaneously a considerable 

 portion of the eastern part of the United States. 



In common with nearly all the commotions of the atmos- 

 phere on the North American continent, they move east- 

 ward, at the rate sometimes of thirty-five miles an hour. In 

 some rare instances the horizontal axis of the storm in a 

 north and south direction is nearly a continuous straight 

 line, and moves side foremost toward the east, in the form of 

 an immense wave, or rather undulation. The pressure on 

 the middle of this wave, on account of the uprising air, is 

 less than the normal pressure of the atmosphere, while on 

 either side, and particularly on the east, it is greater. 



This pressure on the front and rear of the storm is due to 

 the spreading out above of the air which has been carried 

 up in the ascending current, and is greater on the east side 

 of the storm on account of the action of the westerly current 

 in which the whole commotion is carried forward. The 

 approach of the storm is therefore generally indicated by a 

 rise of the barometer, which is succeeded by a subsequent fall^ 

 and also by an increase of temperature due to the radiation 

 from above of the latent heat evolved, and also by the in- 

 creased pressure of the air forced out above. Sometimes the 

 horizontal axis of the storm is curved, and again, which is 

 of more frequent occurrence, broken up into a number of 

 separate parts, forming altogether a system of which the sev- 

 eral portions slightly vary in direction and velocity in their 

 motion to the east. 



These great storms, though of the same general nature as 

 the thunder storm, are attended with an entire subversion 

 of the upper and lower strata of the atmospheric ocean. 

 After one of them has swept over the continent the commo- 

 tion is immediately succeeded by a westerly wind, a great 

 reduction of temperature, and a great increase in the degree 

 of dryness of the air. We have endeavored to give an idea 

 of the motions of the strata of the atmosphere accompanying 

 these changes in Fig. 11, which exhibits an imaginary sec- 

 tion of the currents in an east and west direction. 



