158 ON THE STORM EXPERIENCED THROUGHOUT THE UNITED STATES 



iliary. Can the third cause be the true one? We have, in the present case, a 

 warm current from the south, and, without doubt, a little above it, a cold cur- 

 rent from the west. The consequence would be, a certain amount of precipi- 

 tation from the partial mingling of these two currents where they came in con- 

 tact. The whole amount of air which could be cooled in this way is very 

 small, and the consequent precipitation wauld be only a small fraction of that 

 which was observed. Let us, then, inquire if the fourth cause could be sup- 

 posed to operate. We have already shown from the observations of the wind, 

 without the aid of any hypothesis, that the southerly current must have 

 disappeared by being elevated into the upper regions of the air. This I 

 regard as a necessary deduction from the observations. We find, then, a 

 warm current suddenly cooled, and its moisture must, of course, be in 

 part precipitated. I find by computation, that if the entire atmosphere, satu- 

 rated with moisture at the temperature of 70°, be cooled 5°~ 



" 60 " 7 one inch of 



" 50 " 10 Iwater will be 



" 40 " 16 precipitated. 



" 30 " 25 J 



But the average amount of water which fell throughout the United States 

 was somewhat less than one inch. The cause we have assigned, then, seems 

 adequate to account for the phenomena. We have simply determined, as yet, 

 that the south-east wind must have disappeared by being lifted from the earth's 

 surface. But in what direction did it continue to move ? It might be sup- 

 posed to continue on its course towards the north-west, above the other cur- 

 rent, or to return towards the south-east. Being a current of more than a thou- 

 sand miles in breadth, we can hardly suppose it to escape either by the north- 

 east or the south-west. Did it, then, continue its direction towards the north- 

 west? We should then have a warm and moist current flowing at a great 

 elevation over an exceedingly cold one. The consequence would be, that its 

 moisture would not only be precipitated, but frozen, and would descend to the 

 earth in the form of snow or hail. This is conformable to observation. Snow 

 and hail did fall at nearly all of the northern stations, after the north-west wind 

 set in. But the amount was small; much less than must necessarily result if 

 this entire southerly wind had flowed over the northerly, and had its moisture 



