36 



MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 

 Table XVIII. — American storms advwncing southeasterly. 



center could not be distinctly traced. This example appears to illustrate the general character of 

 areas of low pressure, and shows that this progressive movement is not due to a simple drifting 

 of the atmosphere, but rather to a diminution of ]>ressun' on one side of the low area and an in- 

 crease of pressure on the other side. In the present ease there was scarcely any a])preciable 

 diminution of pressure on the south side, and only a slight increase of pressure on tlie nortii side. 



The low area, No. 17, near the end of its course, exhibited similar peculiarities, and they were 

 the result of similar causes. On March 12. at 3 p. m., there was a well marked low center (29.76) 

 in the northwest part of Georgia, aud a high center (30.83) existed iu Dakota. At 11 P. M. this 

 high area had advanced eastward, the low center had been crowded southward, and the pressure 

 at the low center was 30.04. The next morning the i)ressure throughout Georgia and Florida had 

 further increased, and the low center was crowded still further towards the southwest. 



It will be seen from column 7 of the table that in fill of the cases except four or five there 

 was an area of decidedly high pressure on the north side of the low area, and in several of the 

 cases the influence of this high area was similar to that already noticed iu Nos. 12 aud 17. The 

 low area was crowded southward, and the depression gradually closed up, and became nearly or 

 quite extinct. In some of the remaining cases in which the depression did not close up the high 

 pressure on the north side was apparently the cause which crowded the low area so far to the 

 southward. 



53. The low areas enumerated iu the preceding table were generally followed by a strong wind 

 from the north or northwest. This will be seen from the following table, which shows the height 

 of the barometer, with the direction and force of the wind, at five stations in several cases in which 

 the low centers passed nearest to Galveston, in Texas. 



