290 WRITINGS OF JOSEPH HENRY. [1855- 



latent heat from portions of air charged with moisture. Ac- 

 cording to this view, an intervening region ahnost entirely 

 without moisture will of necessity tend to intercept the pro- 

 gress of a storm, though it is not impossible that the draw- 

 ing in of air on one side of a mountain of limited extent may 

 cause a current across the mountain to supply the defi- 

 ciency. 



We think all the phenomena of the storms of the interior 

 of this continent may be referred to disturbances in the equi- 

 librium in the upper and lower strata of air. In the first 

 place, all the disturbances of the atmosphere, however they 

 may be produced, tend to move eastward over the United 

 States, because this is the resultant motion of the great mass 

 of current passing over the surface of this region. That the 

 storms from the interior tend to move nearly east, with a 

 velocity of from 20 to 30 miles an hour, is abundantly 

 proved by the observations collected at the Smithsonian 

 Institution, and the fact is interestingly and practically 

 exhibited by means of the daily despatches gratuitously 

 furnished this Institution by the Morse line of telegraph. 

 These despatches are received every morning from the 

 greater portion of the country east of the Mississippi River 

 and to render the information available in the way of pre- 

 dicting probable changes of the weather during the day or 

 the following evening, a large map, containing merely the 

 names of the places of observation, is attached to a wooden 

 surface, into which, at each place, a projecting iron pin is 

 driven. Small cards (previously provided) of about an inch 

 in diameter, of different colors, to indicate rain, snow, clear- 

 ness, and cloudiness, are attached to the map at the respective 

 places of observation by means of the iron pins, and changed 

 daily to correspond with the telegraphic despatches, so that an 

 observer, at a glance, may see the condition of the weather at 

 any portion of the country before mentioned. During the 

 autumn, winter, and spring, if in the morning the visitor to 

 the Institution observes a black patch indicating rain at Cin- 

 cinnati, he may conclude that, in about twelve hours after- 

 ward, the same storm will reach Washington. Indeed so 



