44 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



lu No. 16 there was a second low area advancing from the northwest, with which No. 16 coa- 

 lesced December 28. No. 17 was similar to the preceding. These two numbers may, tlereiore, 

 perhaps be transferred to Class I. 



No. 9 was attended by considerable fall of rain in Texas, and this case may perhaps be trans- 

 ferred to Class II. 



No. 4 was attended by an area of high pressure on the east side and a second area of high 

 pressure on the west side of the low center, and between these two high areas the isobars were 

 very much elongate<l towards the southwest. This case may, therefore, perhaps be transferred to 

 Class III. 



The eight remaining numbers show a remarkable agreement in their general features. The 

 dej)ression at the center of the low area did not in cither of these cases increase during the con- 

 tinuance of the westerly movement, and generally the low became decidedly more shallow. 



No. 10 bears a decided resemblance to the cases in Class IV. There prevailed on March 10 a 

 high area (30.40) on the Atlantic Coast; a second high (30.20) on the Pacific Coast ; and a third 

 high (30.60) near Hudson's Bay. The latter high area pushed downward towards the southeast, 

 and this ai>peared to be the cause which crowded towards the west the low center which existed 

 near the Missouri Eiver. There was some fall of rain and snow, but this alone does not seem to be 

 sufiQcient to account for the abnormal movement of this low center. 



59. I next sought for cases in which storms in the middle latitudes of the Atlantic Ocean 

 or Europe have for a day or moi-e pursued a westerly course. For this purpose I have carefully 

 consulted Hoflmeyer's daily weather charts from December, 1873, to November, 1876; also the 

 daily charts of the Danish Meteorological Institute and the Deutsche Seewarte from December, 

 1880, to August, 1881 ; also tlie monthly maps of storm tracks published by the Deutsche See- 

 warte from 1876 to 1884, and the monthly maps of storm tracks according to the international 

 observations from November, 1877, to April, 1882. Table XXYI II shows the most decided cases 

 of these westerly movements. These abnormal movements are represented on Plate XVI, and are 

 designated by the same numbers as in the table. 



60. We perceive from this table that these abnormal movements occur in all mouths of the 

 year, but are most frequent in spring. We also perceive that movements towards the northwest 

 are more than twice as fre(]uent as those towards the southwest, whereas in the United States 

 southerly movements are most common. The average velocity of progress of storm centers during 

 their westerly movement is 15.5 miles per hour, which accords very closely with the average 

 velocity of all storm centers for Euroi)e. 



If we carefully examine each case in the preceding table we shall find examples of three, if 

 not all, of the classes already described for the United States. In nearly every case we find a fall 

 of rain or snow in the region toward wbicli the low center advanced, and in most of the cases the 

 rainfall was unusually great. In the latter statement are included Nos. 0, 9, 15, 16, 17, 18, 19, 

 20, 22, 30, 34, 37. and 41. This list does not include cases in which the low center was over the 

 Atlantic Ocean, since in these cases the amount of rainfall is unknown. It may be inferred from 

 this comparison that a fall of I'ain or snow is one of the most important causes which determine 

 the abnormal movements of areas of low pressure. 



In a large number of the cases in Table XXVIII the low center appeared to be attracted 

 towards a second low center which was at no great distance, and in several cases the two low 

 centers subsequently coalesced. Nos. 1, 2, 3, 4, 5, 7, 8, 9, 11, 12, 13, 14, 18, 19, 20, 23, 24, 26, 28, 

 29, 30, 31, 34, 35, 37, 40, and 41 exhibit this attractive influence of a second low center, and in 

 Nos. 1, 2, 5, 8, 11, IS, 23, 28, 29, 30, 34, 35, 40, and 41 the two low centers coalesced. The mutual 

 influence of two low centers would thus seem to be even more efdcieut than that of rainfall, but 

 these two causes generally concur ; and the correspondence between the two lists of numbers here 

 given would i)robably be more complete if the observations had I'urnished a full report of the rain- 

 fall in the neigliborhood of each low center. 



The number of cases in which the low center advanced between two areas of high pressuie, 

 not very remote from each other, or in a direction lying between two such areas and apparently 

 under their influence, is very great. Nos. 1, 2, 3, 4, 6, 7, 10, 11, 14, 10, 17, 18, 19, 21, 23, 27, 29, 36, 



