60 



MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



87. The cases enumerated iu this table are all comprehended between latitude 40° and lati- 

 tude 72°, and between longitude 66° W. and 52° E., but they are by no means uniformly distrib- 

 uted over tliis area. Eleven cases occurred near the southern extremity ot Greenland, viz, Nos. 1, 

 2, 13, 21, 22, 25, 26, 29, 31, 40, and 41. Nine cases occurred near the western coast of Iceland, viz, 

 Nos. 8, 10, 1 0, 20, 27, 28, 30, 36, and 37. These cases seem to indicate distinctly the influence of local 

 causes. Both of these localities are remarkable for a larger rainfall than is generally found in the 

 same latitude, and it seems probable that this rainfall exerted an influence to hold the low center 

 for twenty-four hours in a nearly fixed position. A similar remark applies to Nos. 5, 7, and 32, near 

 Scotland; to Nos. 6 and 43, on the coast of Norway; to No. 33, near Nova Scotia; and No. 48, 

 near Newfoundland. In all of these cases (twenty-seven in number) it seems probable that local 

 causes exerted an appreciable influence on the movement of the low center, and there are a few 

 other cases in which there is room to suspect the influence of local causes. 



88. There remains, however, a large number of cases situated in the middle of the Atlantic 

 Ocean which we cannot ascribe to anj local influence, and therefore it may be presumed that in the 

 cases above specified local influence was not the sole cause of the slow movement of the low centers. 



It seems impossible to avoid the conclusion that the extremely slow progress of storm centers 

 which we sometimes observe, and the extremely rapid progress which we find at other times, are 

 mainly due to variations in that general movement of the atmosphere which is shown in the 

 average system of atmospheric circulation. It has already been stated that there are permanent 

 causes in operation, which, in the middle latitudes, give rise to an average wind from west to east. 

 The operation of these causes is temporarily suspended by the action of great storms, which give 

 rise to easterly winds, but the permanent causes which influence the winds are not changed by 

 the action of storms, however violent. By temxjorary obstruction the permanent causes acquire 

 increased energy. Hence it results that the general" system of circulation of the winds, although 

 pretty uniform when we compare the average direction of one year with another, appears very 

 irregular when we comjjare one day with another. Sometimes over large portions of the earth's 

 surface the movement of the winds from west to east goes on with destructive violence ; some- 

 times, over extended districts, the movement is reversed, and the prevalent wind is from east to 

 west; while at other times the advance from west to east is almost entirely suspended, or proceeds 

 in the average direction with inconsiderable velocity. According to this view the general system 

 of atmospheric circulation (consisting of the trade winds of the equatorial regions and the 

 prevalent westerly winds of the middle latitudes) is the primary cause which determines both the 

 direction and velocity of the movement of storm centers; but for each individual storm the 

 determining cause is not so much the average system of atmospheric circulation as the general 

 movement of the atmosphere which is going on at the time and in the vicinity of that particular 

 storm. The iufluence of this general movement is moreover materially modified by a variety of 

 causes, such as the amou/it of rainfall and the position of the rain-areas with reference to the 

 center of the storm, the magnitude and position of the neighboring ai"eas of high and low pressure, 

 the distribution of temperature, local influences, &c. 



8!). The preceding investigation has shown that the causes which produce unusually rapid 

 movements or unusually slow movements of storm centers in Europe are similar to the causes 

 which produce like results iu the United States, but it does not explain wby the average movement 

 of storm centers in the United States is so much greater than it is in Europe. In the hope of 

 obtaining some light on this question I have determined the average velocity of storm centers 

 over the Atlantic Ocean by a comparison of the monthly charts of storm tracks i)ublished with 

 the International Bulletin for a period ot four years from 1870 to 1882. The following table shows 

 in miles per hour the average rate of progress for each month of the year : 



The average velocity for the entire year is 18 miles per hour. 



