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SHOET MEMOIRS ON METEOROLOGICAL SUBJECTS. 



rotation, and if we next observe that at the time when the center of 

 the hurricane passed over St. Thomas, at about 8 o'clock, the pressure 

 of the air there was 316 lines, while at Porto Rico it was 333.3 lines, we 

 shall see that, at a distance of 20 (Danish) miles from the axis of the 

 hurricane, the pressure of the air was 17.3 lines greater than at the cen- 

 ter. But if we further consider the foregoing table of air pressures, it 

 will be seen that, while the hurricane was approaching St. Thomas, the 

 pressure of the air there was, at 4.15 a. ra., equal to the pressure at Porto 

 Eico at 8 o'clocls, and that this pressure of the air was again reached at 

 St. Thomas at 11 a. m., when the hurricane was leaving the island. In 

 the course of C§ hours (from 4.15 until 11 o'clock) the hurricane moved 

 about 40 miles (Danish). Its velocity was, therefore, about 6 miles per 

 hour, and this agrees, in a measure, with au observation made on the 

 island of Porto Kico at 12 o'clock, according to which the center of the 

 hurricane seems to have traversed a distance of 20 miles in about four 

 hours. 



In the annexed Figure 2, I have, according to the forrgoiug table of 



atmospheric pressures, observed on the island of St. Thomas during 

 the hurricane, represented the time of observation, expressed in hours, 

 as the abscissa, and the observed pressure of the air, indicated by 

 the height of the mercury, expressed in lines, as the ordinate to a 

 curve, corresponding to the pressures of the air prevailing during the 

 hurricane. By the aid of the points thus designated, I have subse- 

 quently drawn the two branches of the curve as given, which, judging 

 Irom the entire scries of observations, may be considered as correspond- 

 ing fo the pressures of the air which were really exerted during the hur- 

 ricane; and since I have commenced with the lowest pressure (310 lines) 

 that was exerted while the center of the hurricane wafA passing over St. 

 Thomas, it is evident that the curve thus constructed may be compared 

 with the previously mentioned curve a Oi 6i /?, fig. 1, the equation of 

 which, with rectangular coordinates, is: 



9 



