156 ON THE STORM EXPERIENCED THROUGHOUT THE UNITED STATES 



wind, by friction upon the lower stratum, and by gradual subsidence into it^ 

 tends to impress upon the lower current its own direction. This accounts, in 

 part, for our westerly and south-westerly winds, but would not explain the 

 south-east wind of the storm in question. The true explanation of this phe- 

 nomenon is, I think, found in the fact that the greatest depression of the baro- 

 meter was at some point north of the United States. The greatest observed 

 depression was at Quebec; and it is not improbable that the absolute minimum- 

 is to be found still farther north. We should expect a prevalent tendency of 

 the winds towards this point of greatest depression; that is, in the United 

 States, in front of the storm, we should expect a south-east wind, and to the 

 north of the storm's centre, a north-east wind. I have been unable to obtain 

 any observations for testing the truth of this last conclusion. 



I come now to inquire for the cause of the rain. Rain is always owing to 

 one cause, namely, a sudden reduction of the temperature of the atmosphere 

 below the dew point. We run no risk in always assigning this as the imme- 

 diate cause of rain. But how is this reduction of temperature effected? It 

 may occur in a great variety of ways, among which are the following : — 



I. Radiation. 



II. Warm air coming in contact with cold earth or water. 



III. A warm current coming in contact and mingling with a cold one. 



IV. Air suddenly transported into elevated regions. 



It is doubtful whether the first of these causes ever produces rain, for the 

 reduction of temperature is too gradual. It is very common for the thermo- 

 meter to sink during the night below the dew point of the preceding evening, 

 and without any change of wind; yet not only no rain follows, but the atmo- 

 sphere may remain perfectly transparent the whole time. Such circumstances 

 produce a most copious dew, but seldom if ever rain. 



The second cause is one which is often observed, but ordinarily produces 

 merely a fog. Thus the warm air from the gulf stream rolling over the cold- 

 banks of Newfoundland produces the densest fogs; and in winter, the air from 

 the sea flowing in upon the frozen earth has its moisture abundantly condensed. 

 Yet the reduction of temperature effected in this way is also so gradual, it is 

 believed never to produce any considerable rain. 



The third cause, though very similar in principle to the second, is believed 

 to be more efficient than either of the preceding, because it will produce a 



