22 THE EUROPEAN STORM OF DECEMBER 21-28, 1836. 



precipitated descends in the form of snow. The heat liberated in the condensation 

 of this vapor, expands the npper stratum of air, causing it to rise above its usual 

 elevation, and thus to flow off above, causing a diminished pressure at the earth's 

 surface, so that near the centre of the storm the barometer stood about three-fourths 

 of an inch below its mean height. This point of least pressure was situated nearly 

 over the Alps of Switzerland. The low state of the barometer in Switzerland 

 caused the air to press inward from every point of the compass. It was quite 

 impossible that the air should advance in straight lines towards the point of least 

 pressure ; but there was a general circulation of the air around the central point, 

 with a crowding of the air inward towards the centre. The air thus accumulated 

 over the area of least pressure must have risen from the earth's surface, and thus 

 lifted still higher the westerly wind which formed the upper current, causing an 

 increased precipitation, and consequently diminished pressure at the centre of the 

 storm. As the wind which pressed down from the north crosses parallels of lati- 

 tude whose diurnal motion eastward is greater than its own, it has a relative motion 

 to the westward, and thus on the northern border of the storm the north wind 

 becomes a northeast wind. For a like reason, on the southern border of the storm, 

 the southerly wind becomes a southwest wind. Thus, the rotation of the wind 

 takes place in a direction contrary to the motion of the hands of ^ watch. 



A portion of the air which flowed off above from the region of low barometer 

 probably spread northward, and increased the pressure in the north of Europe, thus 

 adding new energy to what was doubtless the primary cause of the storm. 



On the 25th, throughout the Avhole of Western Europe, the phenomena were 

 nearly the same as on the 24:th ; but near the centre of the storm the barometer 

 had fallen still lower, the depression now amounting to about one inch below the 

 mean. On the north of Europe the high barometer still continues, and the line of 

 one-half inch pressure above the mean has advanced steadily to the southward since 

 the first commencement of the storm ; so that while in North Italy the barometer 

 is one inch below the mean, at Christiania, in Norway, it is four-fifths of an inch 

 above the mean. 



On the 26th, throughout Western Europe, the phenomena are nearly the same 

 as yesterday ; but the action of the wind appears to be less uniform, so that while 

 in some places the rain and snow are more violent than yesterday, at other places 

 they have ceased entirely. 



On the 27th the phenomena continue quite similar to those of the 26th ; but 

 there is a general diminution in the violence of the storm, and the greatest depres- 

 sion of the barometer scarcely amounts to two-thirds of an inch. This diminution 

 of violence is probably due to the general prevalence of the cold current from the 

 north, and the diminution of the westerly wind from the ocean, which furnished 

 most of the vapor of precipitation. 



On the 28th we find the same causes operating Avith an energy still further 

 impaired, and probably from the action of the same cause, viz., the failure of the 

 upper current, which furnished the vapor of precipitation. After the 28th the 

 operation of the same causes grew gradually feebler, until at length they disap- 

 peared entirely. 



