180 PHILOSOPHY OF STORMS. 



storm, commencing just before the north west wind set in. 

 The north west wind was also very violent probably about 

 forty or fifty miles an hour, which seems to be about the 

 velocity with which the storm moved while passing from 

 Springfield, Ohio, to Philadelphia. 



140. The central line of this storm also was not directed 

 exactly north and south, but east of north and west of south. 

 It passed Stroudsburg, in the N. E. corner of Pennsylvania, 

 ten minutes before it reached Lancaster, sixty miles west of 

 Philadelphia. It was about an hour later reaching Snow 

 Hill, Md., than it was in reaching Stroudsbnrg. 



141. Here was a most remarkable vein of snow several 

 hundred miles long from N. by E. to S. by W. and only a 

 few miles wide, moving side foremost, and snowing only a 

 very short time at any one place, half an hour at Philadel- 

 phia and Stroudsburg, and still less at Snow Hill and Lan- 

 caster. 



This vein of snow was preceded by a fall of the barome- 

 ter, and succeeded by a very sudden rise, of about half an 

 inch in a few hours, and of three fourths in twenty-four hours. 



It was also succeeded by a sudden fall of the thermome- 

 ter amounting to 20 and 30 in a few hours. 



The difference of barometric pressure on the east and 

 west side of this vein of snow caused no doubt by the dif- 

 ference of temperature, was sufiicient to produce the violent 

 westerly wind, on the west side of the vein of snow : and 

 as the air in front of the vein was calm or nearly calm un- 

 til within a few minutes before the westerly wind com- 

 menced with violence, it is manifest, the air east of the vein, 

 on the approach of the westerly gale, which indeed was 

 the most violent in five minutes after it commenced, must 

 have ascended ; and when it reached a certain height, it 

 would form a cloud and give out its latent caloric of elas- 

 ticity and of fluidity, which would expand the air in the 

 cloud, after allowing for the condensation of the vapor nearly 



