48 



MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



storm center advances. In the United States storms almost invariably advance eastward, and 

 generally towards a point a little north of east; but the wind which presses upon the rear 

 generally comes from the north or northwest, which direction is often at right angles, or nearly 

 at right angles, with the direction in which the storm center advances. Plates I and II exhibit 

 this fact, and the same is substantially true of nearly every great storm shown on the Signal" 

 Service maps. Tiiis movement of the air by which the center of least pressure is carried forward 

 bears some analogy to tlie movements which cause the advance of a wave upon the surface of the 

 ocean, and hence we may with propriety say that the progressive movement of a storm area is the 

 movement of a great atmospheric wave. 



67. Besides the general considerations here stated there are various special phenomena which 

 indicate that the movement of areas of low pressure cannot be fully explained by the theory of 

 a general drift of the atmosphere. We frequently find two neighboring low areas advancing 

 in directions inclined to each other at an angle of 45°, or even a greater angle. In the United 

 States, while a low center is advancing from Florida along the Atlantic coast, towards the north- 

 east, another low center may be advancing eastward over the region of the Great Lakes, and the 

 two low centers may coalesce somewhere in the neighborhood of Nova Scotia or Newfoundland. 

 It will be seen from Plate X that the storms which proceed from the Gulf of Mexico, and from the 

 neighborhood of the West India Islands, generally advance towards Newfoundland, and the storms 

 which come from the northwestern part of the United States also tend towards the same region. 

 Newfoundland becomes thus a point of convergence of storm tracks, proceeding from regions quite 

 remote from each other. In the vicinity of Newfoundland there exists some influence which ap- 

 pears to act as an attractive force upon storm centers. This influence probably results from the 

 great amount of precipitation near that island, arising from the i)roximity of the warm water of 

 the Gulf Stream to the colder air from the land. Plate XI shows other points towards which 

 storm tracks seem to converge, particularly the Asiatic coast near Japan, and this fact probably 

 results from a cause similar to the one just named. If Plates X and XI exhibited the storm tracks 

 of different regions according to the relative frequency of their occurrence, other points of con- 

 vergence of storm tracks would be exhibited. Along these converging storm paths two storms 

 often travel simultaneously and coalesce in a single storm area. Such a movement appears incon- 

 sistent with the drift theory. 



68. For the convenience of those persons who may wish to investigate cases of this kind for 

 themselves I present the following list, which shows some of the most decided cases in which two 

 centers of low pressuie in the United States have coalesced. They are taken from the Signal 

 Service weather maps for the years 1873-18S0. These maps show a considerable number of other 

 cases of like kind, some of which have been omitted because the depression of the barometer was 

 small, and others because the position of the low center was not very sharply defined, or was 

 situated near the margin of the weather map: 



Examples in which two centers of low pressure approach each other and coalesce. 



Among these twenty-four cases there are only three in which the paths of the two low centers 

 wei'e not inclined to each other at an angle as great as 45°; in half of the cases the two paths 

 were inclined at an angle considerably greater than 45°; in eight or nine of the cases the angle 

 was nearly as great as !)0O; and in three of the cases the angle was greater than 90°. 



69. It sometimes happens that within an area of low pressure, having but a single center, a 

 second low center is developed. The following list shows twenty- four such cases selected from the 

 Signal Service maps for 1873-1880, The maps show a large number of other similar cases; but in 



