10 THE AMERICAN STORM OF DECEMBER 20, 1830. 



caloric liberated in the condensation of the vapor, and this probably occasioned the 

 rise of the thermometer in front of the storm, which Avas particularly noticeable on 

 the morning of the 21st. 



From the evening of December 19th to the evening of the 21st the phenomena 

 remained sensibly the same, except that the storm had drifted steadily eastward, 

 being borne along by the strong west wind ; and since this west wind was more 

 violent than the southeast Avind, the partial vacuum prevailing under the storm was 

 gi'adually filling up, and after two or three days more it had nearly disappeared. 



What was the Cause of the extreme Cold which succeeded this Storm? 



On the morning of December 20th, near St. Louis, the thermometer stood at its 

 mean heiglit. At the same place, on the evening of the same day, the thermometer 

 had fallen to thirty degrees below the mean ; being a descent of thirty degrees in 

 twelve hours, independent of the diurnal variation. 



This cold cannot be ascribed to radiation, for the change took place under the 

 cloud, and even before snow had entirely ceased falling. A portion of the effect 

 may be ascribed to the transfer of air from a higher to a lower latitude. The wind 

 had blown with a probable velocity of sixty miles per hour for twelve hours, during 

 which time it must have advanced 720 miles towards the southeast, or about 500 

 miles in the direction of a meridian, which is equivalent to about seven degrees of 

 latitude, and corresponds to a change of temperature of about eighteen degrees. 

 But a portion of this cold would be lost during the transfer to a warmer latitude, 

 so that we cannot ascribe to a change of latitude an effect greater than twelve or 

 fifteen degrees. 



This cold air being somewhat warmed during its progress southward, is thereby 

 rendered relatively dryer ; and this dryness accelerates the evaporation of the mois- 

 ture upon the earth's surface. This evaporation begets cold on the evaporating 

 surface ; and this cold is shared in part by the air in contact with the earth's sur- 

 face. This cause must tend in some degree to check the increase of temperature 

 which the air would otherwise experience in its transfer to a warmer latitude ; but 

 I do not ascribe to this cause any powerful effect. There still remains to be 

 accounted for a depression of the thermometer amounting to about fifteen degrees. 



I believe that this cold current came from the upper stratum of the atmosphere ; 

 and that the upper part of the atmosphere, Avhen brought down to the level of the 

 sea, is generally colder tlian the average temperature of the earth's surface. Ac- 

 cording to the theory of Dalton, the atmosphere can only be in a state of equilibrium 

 when a pound of air, at all distances above the earth's surface, contains the same 

 quantity of heat. Under sucli circumstances, if air be brought down from the top 

 of a mountain, by coming under increased pressure it must acquire exactly the 

 temperature of the air which was previously at the base of tlie mountain. But 

 such an equilibrium is seldom, if ever, attained in nature. The heat which is 

 liberated during a great fall of rain or snow increases the temperature of the upper 

 air; and if this air could be suddenly brought down to the earth's surface, it might 



