52 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



Rate of progress of areas of low pressure. 



7,3. Ill order to exhibit the average velocity with which centers of low pressure advance over 

 the United States, I have prepared Table XXIX, which shows, in miles per hour, the average 

 velocity of storm centers for each month during a period of thirteen years, according to the 

 observations of the United States Signal Service. 



We se« from this table that the average velocity of progress of storms for the entire year is 

 28.4 miles ; also that the velocity is greatest in February and least in August, and that the former 

 velocity is 50 per cent, greater than tlie latter. We also see that the velocity varies very much 

 for the same month in different years, the greatest mean velocity for several of the months being 

 nearly double the least mean velocity for the same months. 



74. In order to discover, if possible, the cause of these unequal movements, I have selected 

 the most remarkable cases of extremely rapid motion, and also the cases of extremely slow motion, 

 during the period for which the tri-daily observations of the Signal Service have been published, 

 viz, from September, 1872, to January, 1875, inclusive, also for the year 1877, making in all forty- 

 one months of observations. Table XXX shows the cases in which a storm center has advanced 

 at least 1,000 miles in twenty four hours. 



Column 1 gives the number of reference; column 2 the date at which the rapid motion 

 commenced, where the figures 1, 2, and 3, attached to the day of the month, denote the first, 

 second, and third of the hours of observation foi- the given day; column 3 shows the station at 

 which the barometer, at the given date, was lowest ; column 4 the height of the barometer at the 

 station named ; column 5 shows the station at which the barometer was lowest twenty-four hours 

 after the date given in column 2; column 6 the height of the barometer at the station named in 

 the preceding column; column 7 shows in miles the movement of the low center during twenty-four 

 hours, as indicated by the isobars, which best represent the Signal Service observations ; column 

 8 the change in the barometer at the stations named in column 3, during the day here considered; 

 (+ denotes increasing i^ressure, — denotes decreasing pressui-e) ; column 9 the change in the 

 •barometer at the stations named in column 5 during the same day ; column 10 shows the average 

 rain-fall at all the stations within the low area (determined by the isobar 30 inches), on the east 

 side of the low center, for each period of eight hours during the given day. These numbers should 

 therefore be multiplied by three, in order to show the average rainfall for the day in question ; 

 column 11 shows the average rain-fall at all the stations within the low area on the west side of 

 the low center, for each period of eight hours ; column 12 shows the average velocity of the wind 

 (in miles per hour), at the stations within the low area on the east side of the low center; and 

 column 13 shows the average velocity of the wind at the stations within the low area on the west 

 side of the low center. Generally the retreat of the low area eastward was immediately succeeded 

 by an area of high pressure on its western side. Column 14 shows the highest pressure observed 

 at any station within this area of high pressure; and when there was no succeeding area of high 

 pressure, it shows the highest pressuie immediately succeeding the area of low pressure. At the 

 bottom of the table are given the means of the inimbers in ten of the columns. 



75. Table XXXI shows the cases in which a storm center has advanced not more than 240 

 miles in twenty-four hours. The arrangement of this table is similar to that of Table XXX. The 

 Signal Service observations show a considerable number of other cases, which, perhaps, ought to 

 be included in these tables, but which are omitted on account of the uncertainty respecting the 

 exact i)osition of the center of low pressure. 



76. The following are some of the results derived from a comparison of these two tables : 



1. For the cases in Table XXXI the average pressure at the low center was the same at the 

 close of the given day as at its beginning ; that is, the storms neither increased nor diminished in 

 intensity. For the cases in Table XXX the average pressure at the low center was 0.20 in<-h less 

 at the close of the given day than at its beginning ; that is, the storms increased considerably in 

 intensity during the day in question. 



2. The average rate of progress of the storms named in Table XXX was more than seven 

 times as great as that of the storms named in Table XXXI. 



3. In the cases named in Table XXXI the barometer fell, on an average, 0.0!) inch in twenty- 

 four hours in front of the storm, and rose 0.09 inch in the rear of the storm. In the cases named iu 



