40 



MEMOIES OF THE NATIONAL ACADEMY OF SCIENCES. 



movement; colninn .3 the duration of the westerly nicvemeut, expressed iu units of eight hours; 

 columns i and 5 the latitude and longitude of the points of beginning and end of the westerly 

 movement; column 6 the prevalent direction of this movement; column 7 its velocity in miles 

 per hour; column 8 the lowest barometer reported during the continuance of this westerly move- 

 ment; and column 9 shows its subsequent course. These abnormal movements ai'e all delineated 

 on Plate XV, and are designated by the same numbers as in the table. 



Table XXII. — Areas of low pressure in the United States, advancing westerly. 



58. We perceive that these cases are distributed with tolerable uniformity through the diflerent 

 months of the year ; they are not restricted to any particular portion of the United States, aud 

 their average velocity of progress is 23 miles per hour, which is somewhat less than the average 

 velocity of the storms of the United States. If we seek in each case for the probable cause of its 

 abnormal movement we shall be forced to conclude that this cause was not the same iu all cases. 

 Among these causes we notice the following : 



I. Sometimes near to an area of low pressure, with its system of circulating winds, we find a 

 second area of low pressure, having also its own system of circulating winds. Between these two 

 low centers the winds are usually feeble, and they sometimes change into a single system of winds 

 circulating about a low area of an elongated form. If the depression continues the area of low 

 pressure usually becomes less elongated and the two low centers coalesce. By this union the 

 western low center is accelerated eastward, and the eastern low center is temporarily diverted 

 towards the west. Table XXII affords several illustrations of this principle. 



No. S was at first a small dejiiession near Charleston, S. C, and was advancing slowly north- 



