58 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



center is not clearly iudicated, tiiid in other cases the change in the form of the isobars in twenty- 

 four hours was so great as to leave some doubt respecting the identity of the low areas. With 

 regard to tlie rapid movement of the twenty -nine cases included in the table it is thought there 

 can be no difference of opinion. 



Column 1 gives the number of reference ; column 2, the dates of the two maps compared ; 

 columns 3 and 4 show the latitude and longitude of the low center at the first of the two dates ; 

 column 5 shows the estimated heiglit of the barometer in millimeters at the center of the low 

 area. This estimated pressure is generally two or three millimeters less than that of the lowest 

 isobar drawn on the map. Columns C and 7 show the latitude and longitude of the center of the 

 low area at the second of the two dates, and coluiuu 8 shows the estimated jtressure at the center; 

 column 9 shows the i)rogress of the low center in tweutyfour hours expressed in English miles; 

 column 10 shows the rise of the barometer at the first-named point during the twenty four hours 

 succeeding the first dates, and column 11 shows the fall of the barometer at the second-named 

 point during the twenty-four hours preceding the last named date. Generally the low area was 

 immediately succeeded by an area of pressure above 760"'". Column 12 shows the highest isobar 

 in this area. The highest pressure was probably a few millimeters greater than the highest 

 isobar. At the bottom of the table are given the average values of the numbers in six of the 

 columns. 



83. We see from this table that the depression at the center of the low areas increased slightly 

 during the given twenty-four hours, showing a slight increase in the inteusity of the storms. The 

 average rise of the barometer at the first station during the succeeding twenty-four hours was 

 23.2""", or 0.91 inch, and the average fall of the barometer at the second station during the same 

 time was 20.6""", or 0.81 inch. These changes are nearly one-half greater than those shown in 

 Table XXX for American storms. 



If now we examine each of these cases singly we shall find that in about half of them, viz, 

 Nos. 4, 9, 11, 12, 13, 15, 16, 17, 22, 23, 24, 25, 26, and 27, there was a second low center nearly in the 

 direction towards which the first low was moving, and this may be supposed to have accelerated 

 the movement of the first low center. The same was probably true in several cases not distinctly 

 indicated by the charts, and in all of the cases the point reached by the low center at the end of 

 twenty-four hours (if not previously the center of a system of circulating winds) was at least a 

 point where tlie pressure was less than 760""", and the barometric gradient was very feeble. No. 

 14 may perhaps be claimed as an exception to this remark, but if we had a chart for the evening 

 of January 27 it might perhaps appear that the low area prevailing in the middle of the Atlantic 

 Ocean on the morning of January 28 is to be connected with the low area prevailing south of 

 Hudson's Bay on the morning of January 27. 



In more than a third of the cases, viz, Nos. 6, 7, 9, 10, 12, 15, 16, 17, 23, 25, and 26, the low 

 center advanced between two neighboring areas of hij^h barometer, and its movement may have 

 been thereby accelerated. The same was probably true in some cases not indicated by the maps, 

 since the low areas enumerated in the table were geu<M-!illy near the northern limit of the charts. 

 It uuist, however, be admittetl that in many of the e:ises the charts do not show two such areas 

 of high barometer. In all of the cases the charts show a fall of rain or snow on the front side of 

 the low center, but I have not been able to make any satisfactory estimate of its amount. As 

 the low center moved forward it was succeeded h\ a i)ressure above 760""", as is shown by 

 column 12 of the table. The average of the highest isobars, following the low centers, was 

 770"™, and the average pressure at the center of the high areas was about 772"""', or 30.39 inches, 

 the same as found in Table XXX. 



86. Table XXXV shows the most distinctly marked cases in which a storm center, over the 

 Atlantic Ocean or Euroi)e, has advanced not more than 200 miles in twenty-four hours. The 

 arrangement of this table is similar to that of Table XXXIV. 



We see from this table that during the twenty-foui' hours here compared the depression at 

 the center of the low area in some cases increased, and in other cases decreased, but generally 

 the change did not exceed two or three millimetei's. On an average of the fifty cases the 

 depression at the center was slightly diminished. In forty of the cases there was a slight increase 

 of pressure at the first staUou during the twenty-four hours here considered, auc| iu thirty-two of 



