54 MEMOIRS OP THE NATIONAL ACADEMY OF SCIENCES. 



4. During the days named in Table XXXI the average daily raiu-fall for tlie entire low area 

 was 0.27 inch on the east side of the low center and 0.09 inch on the west side. During the days 

 named in Table XXX the average daily rain-fall for the entire low area was 0.48 inch on the east 

 side of the low center and 0.18 inch on the west side. In each case the rainfall on the east side 

 of the low center was about three times as great as on the west side, and for the storms in Table 



XXX the average rain-fall was about double that for the storms in Table XXXI. 



5. For the storms in Table XXX the average velocity of the wind was 3.02 miles per hour 

 greater on the west side of the low center than it was on the east side. For the storms in Table 



XXXI the average velocity of the wind was 2.94 miles per hour greater on the west than on the 

 east side. For the storms in Table XXX the average velocity of the wind was a half mile per 

 hour greater than for the storms in Table XXXI. 



77. We conclude from these results that the velocity of the wind, within an area of low 

 pressure, has very little influence upon the rate of progress of the low center. Moreover, the rain- 

 fall within the low area cannot be the sole cause, and probably is not the principal cause, of the 

 very rai)id ])rogress which the center of low pressure sometimes exhibits, for in sixteen of the 

 cases in Table XXX the rainfall was less than the average rainfall for the cases in Table XXXI, 

 In No. 9 of Table XXX no rainfall w.as reported at any station within the low area either on the 

 east or west side during the twenty-four hours named, and in Nos. 2, 3, 12, 22, 24, and 30 the 

 amount of rain-fall was quite insigniflcant. On the other hand, in seven cases of Table XXXI the 

 rain-fall was greater than the average rain-fall in Table XXX, and in No. 47 the rain-fall on the 

 east side of the low center was greater than in any storm named in Table XXX. 



The facts which the two tables exhibit most strikingly in contrast are that iu the cases of 

 rajiid progress the storms generally were increasing in intensity, and the extent of the oscillatiou 

 of the barometer iu twenty-four hours was almost exactly proportional to the rate of juogress of 

 the storm center. 



78. In order to discover the causes which were most influential in accelerating the movements 

 in Table XXX, and retarding the movements in Table XXXI, I have carefully examined each 

 case, and have found the following results : 



In twenty of the cases in Table XXX the movement of the low center appeared to be accele- 

 rated by the influence of a low area on its east or northeast side, viz, iu Nos. 3, 6, 8, 9, 13, 18, 19, 

 20, 24, 25, 27, 28, 30, 32, 33, 34, 35, 37, 38, and 41. There are also four other cases which appar- 

 ently ought to be included in the same class, but the Signal Service observations do not cover 

 snflicieut territory to furnish decisive information on this point. These cases are Nos. 1, 17, 39, 

 and 40. 



In twenty-two of the cases in Table XXX the low center advanced between two neighboring 

 areas of high barometer, and its movement was apparently accelerated thereby, viz, in Nos. 2, 3, 

 4, 5, 6, 9, 12, 14, 15, 16, 17, 20, 21, 22, 23, 24, 31, 35, 36, 37, 42, and 43. It is probable that No. 26 

 should be included in the same class, and perhaps two or three other cases. 



There are only eight cases not included in either of the i)receding lists, viz, Nos. 1, 7, 10, 11, 

 26, 29, 39, and 40, and seven of the cases are included in both lists. There is little doubt that 

 Nos. 1, 39, and 40 should be included in the first list, but the low center passed so near the 

 northern boundary of the United States that the evidence is not entirely satisfactory. There are 

 only four cases which do not api)arently belong to one of the preceding classes, viz, Nos. 7, 10, 

 11, and 29, and iu these cases the amount of rainfall on the east side of the low center was 

 unusually great, viz, an average rain-fall of 0.81 inch, 0.72 inch, 1.14 inch, and 0.93 inch, iu 24 

 hours, for the entire area within wh ch the barometer was below 30 inches. These are amoirg the 

 greatest rainfalls which have occurred iu the United States since the SigUiil Service observations 

 commenced. Greater rainfalls have occurred within districts of limited extent, but few cases 

 have occurred which showed so large an average rain-fall for the entire extent of the low area. 



In several of the cases in Table XXX the isobars were very much elongated in the direction 

 towards which the low center advanced, so that a small change of pressure was sufficient to carry 

 the low center forward with unusual rapidity. Nos. 4, 7, 10, 14, 17, 21, 23, 30, 34, 36, 37, 40, 

 and 42 were of this kind, and if the stations of observation had been sutficiently extended to show 

 in each instance the complete form of the isobars it is probable that more cases of the same kind 



