Clayton — Six and Seven Day Weather Periods. 229 



ing with a solar rotation, the storm tracks were found in 

 groups, in each of which the cyclones all followed the same 

 general direction, and were separated from each other by 

 intervals of six or seven days, or in some cases by half these 

 intervals. 



During each interval of 27 days one of the periods alone 

 was paramount, either the seven day or the six day, as the 

 case might be, and this period ran through all the groups. In 

 many of the groups the storms followed each other along such 

 approximately parallel tracks that even the irregular bends in 

 the tracks were the same in all. One of the 27 day intervals 

 began daring the first week in May, 1891, and the chart of 

 storm tracks drawn by the United States Weather Bureau for 

 that month is here reproduced as furnishing a good illustration 

 of the phenomena under discussion. On this chart small 

 circles show the position of the storm centers at 8 A. M. and 

 •8 P. M., and the figures above these circles give the days of the 

 month. 



It is seen from this chart that the storms distinctly divided 

 themselves into two groups, one beginning in British America 

 and pursuing the usual course across the great lakes, and the 

 other beginning near the Atlantic seaboard and taking a very 

 unusual course toward the south. It is seen that the succeed- 

 ing storms in each group followed each other so closely that 

 the irregular bends in their tracks are approximately the same 

 in all. But what is of especial interest here is the approxi- 

 mately regular interval of seven days between the succeeding 

 storms. For example in the northern group of storm tracks, 



