2 THE AMERICAN STORM OF DECEMBER 20, 1836. 



stood two-tenths of an inch ahove the mean ; another line connecting all those 

 places where the barometer stood fonr-tenths of an inch above the mean, etc. A 

 line was also drawn connecting all those places where the barometer stood two- 

 tenths of an inch heJow the mean ; another line four-tenths of an inch below the 

 mean, etc. These lines may be Called lines of equal barometric disturbance. 



In like manner a line was drawn connecting all those places where the thermo- 

 meter stood at its mean height for the given hour and month ; and this was called 

 the line of mean temperature. Another line was drawn connecting all those places 

 where the thermometer stood ten degrees above the mean ; and other lines were 

 drawn connecting those places where the thermometer was twenty degrees above 

 the mean ; ten degrees below the mean ; twenty degrees below the mean, etc. 

 These lines may be called lines of equal thermometric disturbance. 



This mode of representing the observations of the barometer and thermometer 

 appears to me the most suitable of any which has yet been proposed, to indicate 

 the connection between the pressure and temperature of the air on the one hand, 

 and the direction of the wind and the fall of rain on the other. 



The results of the investigation of the storm of February 4, 1842, were entirely 

 satisfactory to myself, and were published in the Transactions of the American 

 Philosophical Society, vol. ix. p]x 161-184. Being convinced of the superior 

 advantages of this new method of investigating the phenomena of storms, I imme- 

 diately constructed upon the same principle a scries of maps, representing the 

 progress of the storm of December 20, 1836 ; and I perceived that certain features 

 of this storm were thus brought out more clearly than I had been able before to 

 exhibit them; but their publication has hitherto been delayed by the serious 

 expense attending the preparation of a series of colored maps. As, however, my 

 paper upon this storm has been commented upon by Messrs. Espy, Hare, and Red- 

 field, and some conclusions have been drawn from it which appear to me unwar- 

 ranted, I have thought it important that the maps should be published, with some 

 accompanying remarks sufficient for their explanation. 



Before issuing this reinvestigation of the storm of December 20, 1836, I have 

 employed every means in my power for obtaining additional observations, and have 

 met with some success. I have obtained copies of meteorological journals, including 

 barometric observations, from the following stations, in addition to those employed 

 in my first investigation : — 



About the 25th of December, 1836, a remarkable storm was experienced over 

 nearly the entire continent of Europe, and some meteorologists supposed that this 

 storm was the same as the American storm of December 20th. In order to prose- 

 cute this inquiry, I collected European meteorological journals for this period as 

 far as they were within my reach. These stations were twelve in number, and a 



