132 ON THE STORM EXPERIENCED THROUGHOUT THE UNITED STATES 



The fall of the thermometer is very sensible in all of the observations, and 

 the instant of change I have indicated by a dark line drawn across the table. 

 The observations at the military posts were made at 7, A. M.; 2 and 9, P. M.; 

 and the others were generally made about the same hours. It is not easy to 

 compare, with entire precision, the rate or amount of the depression of the 

 thermometer at the several stations, because the change occurring at different 

 hours is variously affected by the diurnal change of temperature. A correction 

 needs to be applied for this inequality before the observations are comparable, 

 and this correction it is difficult to apply with entire satisfaction. It requires, 

 however, but a hasty inspection of the table to perceive that the depression of 

 the thermometer was greater in the northern than the southern states, and 

 greater in the western than the eastern ; that is, the fall was most remarkable 

 of all in the north-western states. Thus, for example, at Fort Crawford, from 

 the morning of the 20th to that of the 21st, at the same hour, the thermometer 

 fell 50°; at Augusta the same, and at Fort Winnebago 49°. At Augusta, on 

 the 20th, the thermometer fell 38° from sunrise to 2, P. M., that is, against the 

 diurnal variation, which, according to observations made at the same place for 

 the entire month, amounts to ten degrees; making the real depression of the 

 thermometer forty-eight degrees in six and a half hours, the most remarkable 

 fluctuation I recollect ever to have heard of; and this, too, in a latitude south 

 of Naples. 



The commencement of this depression coincided sensibly with the minimum 

 of the barometer. This is very satisfactorily shown by the hourly observations. 

 In a few instances the thermometer fell a degree or two before the barometric 

 minimum ; but in all cases the fall of the thermometer became very rapid after 

 the barometer began to rise. This appears to be a well established feature of 

 the storm, and I have availed myself of it in tracing the progress of the storm 

 in regions where barometric observations could not be obtained. This was par- 

 ticularly true of the north-west part of the United States, and I have projected 

 on Plate I., Fig. 2, the observations made in this region in the same manner as 

 the curves on Plate I., Fig. 1, were constructed. The observations were first cor- 

 rected for the diurnal inequality. The curves exhibit to some extent the effect 

 of local causes; although, on the whole, the resemblance is quite striking, and 

 probably would have been still more so if the observations had been made at 

 shorter intervals. 



