E. Loomis — Areas of low pressure. 



11 



cannot be long maintained. It occasionally happens, during a 

 violent storm, that the east winds are stronger than the west 

 winds. In such a case the low center may be pushed westward ; 

 but such a result does not necessarily follow, for a large part 

 of the air which pushes in on the east side rises from the earth's 

 surface, while the air which pushes in on the west side does 

 not rise at all, or not to an equal extent. Thus the low area is 

 filled up on the west side, and were it not for the continued 

 precipitation of vapor, the low area would soon become oblit- 

 erated. 



Mate of progress of areas of low pressure. 



In order to exhibit the average velocity with which centers 

 of low pressure advance over the United States, I have pre- 

 pared the following table which shows, in miles per hour, the 

 average velocity of storm centers for each month during a 

 period of thirteen years, according to the observations of the 

 United States Signal Service. 





Jan. 



Feb. 



Mar. 



April. 



May. 



June. 



July. 



Aug. 



Sept. 



Oct. 



Nov. 



Dec. 



1872 



31-2 



29-4 



34-5 



34-5 



24-7 



21-8 



24-6 



183 



22-2 



209 



23-6 



28-8 



1873 



25-8 



32-7 



28-1 



22-3 



235. 



20-8 



24-6 



17-8 



23-1 



28-1 



27-9 



26-7 



1874 



23-0 



33-9 



29-8 



31-4 



22-2 



22-4 



25-9 



19-9 



23-1 



28-5 



30-3 



32-7 



1875 



32-1 



32-8 



30-0 



26-4 



29'2 



31-5 



25-3 



17-1 



30-5 



23-4 



30-0 



31-1 



1876 



38-1 



31-5 



26-4 



23-6 



24-7 



19-3 



26-4 



23-2 



23-8 



27-7 



22-6 



383 



1877 



37-7 



26-5 



32-6 



252 



27-3 



252 



24-2 



20-0 



17-4 



20-2 



255 



24-7 



1878 



26-3 



27-7 



243 



22-tt 



17-9 



18-4 



21-7 



26-8 



23-9 



19-6 



212 



340 



1879 



35-5 



333 



35-1 



27-8 



25-3 



29-4 



26-4 



21-0 



21-7 



30-8 



40-7 



38-7 



1880 



37-6 



396 



35-8 



27-2 



25-] 



24-5 



25-7 



25-9 



23-5 



22-3 



34-1 



38-8 



1881 



32-3 



35-4 



26-8 



37-1 



32-6 



32-8 



26-6 



25-4 



30-6 



37-5 



30-8 



33-6 



1882 



421 



4T6 



34-8 



29-5 



216 



26-8 



19-8 



19-9 



23-5 



27-7 



27-7 



30-2 



1883 



39-8 



36-4 



38-0 



28-4 



30-0 



24-2 



25-8 



28-0 



25-0 



37-3 



39-4 



33-0 



1884 



38-6 



43-9 



33-3 



21-5 



26-8 



20-5 



22-4 



30-7 



32-6 



34-4 



35-2 



43-7 



Mean 



33-8 



34-2 



31-5 



27-5 



25-5 



24-4 



24-6 



22-6 



24.7 



27-6 



299 



33.4 



We see from this table that the average velocity of progress 

 of storms for the entire year is 28*4 miles ; also that the veloci- 

 ty is greatest in February and least in August, and that the 

 former velocity is 50 per cent greater than the latter. We also 

 see that the velocity varies very much for the same month in 

 different }'ears, the greatest mean velocity for the months of 

 April and October being more than double the least mean 

 velocity for the same months. 



In order to study the movement of areas of low pressure 

 under the greatest possible variety of circumstances, I have en- 

 deavored to obtain information from European observations. 

 In the Uebersicht der Witterung for 1881, published by the 

 Deutsche Seewarte, is given a table showing the mean velocity 

 of movement of the barometric minima for the five years 

 1876-80, as deduced from the monthly charts of storm tracks. 



