United States Weather Maps and other sources. 15 



The isobar of 29 inohes is shown on the accompanying chart. 

 On the afternoon of the same day, this storm blended -with 

 another storm on its eastern 'side, and there resulted one of the 

 most violent hurricanes ever experienced on the Atlantic 

 Ocean. At 6 p. m. the barometer fell to 27-33 inches, which 

 Capt. Toynbee pronounces the lowest ever observed on this 

 part of the Atlantic. The accompanying chart represents the 

 isobar of 29 inches on the morning of the 6tli, when the diam- 

 eter of this curve was over 1000 miles, and the diameter of the 

 isobar of 30 inches was over 2000 miles. During the next two 

 days the storm advanced slowly towards the southeast, and its 

 severity was much diminished. The accompanying chart shows 

 the isobar 29"5 inches on the morning of Feb. 8th. During 

 this interval of three days the center of the storm had moved 

 only about 900 miles, "showing an average velocity of about 

 12 miles per hour. 



Application of FerreV s formula. 

 In vol. viii of this Journal, p. 343, Prof. Ferrel has given a 

 formula which enables us to compute the depression of the 

 barometer resulting from a violent storm. If we divide the 

 denominator of this formula by the number of inches in a mile, 

 and suppose the wind to move in a circle, the formula becomes 



G = JL^^ + ^^, 

 250 131r' 



where G is expressed in inches, but v and r are expressed in 

 miles. I have applied this formula to the storm of Feb. 5th, 

 1870, and the results are shown in the following table. Col- 

 umn first shows the isobars which have been selected as the 

 basis of comparison ; column second shows the radius of each 

 isobar as nearly as can be determined from the observations of 

 Capt. Toynbee's memoir ; and column third shows the velocity 

 of the wind in miles on each of these isobars. These velocities 

 were obtained by taking the mean of the various observations 

 corresponding to the barometric heights given in column first. 

 These velocities were recorded in the numbers of Beaufort's 

 scale (0-12) and were reduced to miles by the table in Scott's 

 Met Instruments, p. 68. Column fourth shows the gradient to 

 100 miles computed by the above formula, for points midway 

 between the several isobars selected. If this gradient be sup- 

 posed to be maintained for a distance equal to the distance be- 

 tween the isobars, it will show a change of barometric pressure 

 about the same as that actually observed. For the inner circle, 

 the computed gradient will represent the observed depression 

 of the barometer if we suppose that near the center of the 

 storm there was a considerable mass of air revolving with a 

 diminished velocity. 



