E. Loomis— Areas of low pressure. 



13 



and is double the rate of progress for the China Sea ; and the 

 rate of progress for the United States is more than three times 

 the rate for the China Sea. These results are derived from so 

 large a number of observations, that they must be accepted as 

 substantially correct, and they demand a clear explanation. 



I have endeavored to determine how far these differences may 

 result from a difference in the mean velocity of the wind for these 

 several districts. For this purpose I determined the average 

 velocity of the wind for that portion of the United States 

 within which the storm centers are most frequently found, viz: 

 that portion north of the parallel of 40°, and east of the meri- 

 dian of 100° from Greenwich. A slight examination of the 

 observations shows that at stations near the Atlantic Ocean, or 

 near one of the Great Lakes, the velocity of the wind is greater 

 than at stations in the interior of the country. I have there- 

 fore divided the observations into two groups, one including 

 the stations near the ocean, or one of the Great Lakes, and 

 called coast stations (twenty-five in number), the other group 

 including the remaining stations, which are called inland sta- 

 tions (twenty in number) ; the second and third columns of the 

 following table show, for each month of the year, the mean 

 velocity of the wind in miles per hour for these two classes of 

 stations, according to the Signal Service observations : 





Coast 



Inland Mean. 



Storms. 



Ratio. 





Coast 



Inland 



Mean. 



Storms, 



Ratio. 





Sta. 



Sta. 









Sta. 



Sta. 









Jan. _ 



10-98 



8-40 9-69 



33-8 



3-5 



July,. 



7-70 



6-87 



7-28 



246 



3-4 



Feb.. 



11-33 



8-94 10-13 



34-2 



3-4 



Aug. . 



7-42 



6-40 



6-91 



226 



33 



Mar. . 



11-76 



10-18 10-97 



31-5 



2-9 



Sept. . 



8-98 



6-96 



7-97 



24-7 



3-1 



April 



10-67 



9-59 10-13 



27-5 



2-7 



Oct. .. 



9-96 



7-S4 



8-90 



27-6 



3-1 



May _ 



9-22 



8-55 8-88 



25-5 



2-9 



Nov. . 



11-16 



8-50 



9-83 



29-9 



3-0 



June 



8-34 



756 7-95 



24-4 



31 



Dec. _ . 



11-30 



8-20 



9-75 



33-4 



3-4 



The 4th column gives the mean between the numbers in the 

 two preceding columns; the 5th column shows for each month 

 the rate of progress of storm centers ; and the 6th column shows 

 the ratio of the velocity of storm centers to the mean velocity 

 of the wind. 



We see that this ratio is not the same for all months ; but 

 for that month in which the rate of progress of storms is great- 

 est, the ratio is sensibly the same as for that month in which 

 the rate is the least. This coincidence seems to indicate that 

 the rate of progress of storms is in some degree dependent 

 upon the mean velocity of the wind ; but the considerable 

 inequalities in the value of this ratio show that the rate of 

 progress of storms cannot depend solely on the average velocity 

 of the wind. 



I next determined, as well as I was able with the means at 

 my command, the average velocity of the wind for that part of 



