Clayton — Six and Seven Day Weather Periods. 227 



years. In the six day period the figures indicate that the 

 maxima have probably moved backward about half a day in 

 six years. These results show that the mean length of the 

 seven day period as given above should have been increased 

 - 04 hr. and the mean length of the six day period decreased 

 about 0-03 hr. 



The mean height of the barometer at Blue Hill for 8 A. M. 

 and 8 P. m. on each day of the periods was obtained, and the 

 results corrected for the diurnal period by subtracting 0*02 

 inch from the 8 A. M. means were as follows for the three 

 years 1887, 1891, and 1893. These years were selected merely 

 because they were most convenient for the purpose. 



Mean height of the barometer at Blue Hill for each day of the Seven day period. 





1. 2 





3. 



4. 





8 A. M. 8 P.M. 8 A. M, 



8 P.M. 



8 A. M. 



8 P. M. 8 A. M. 8 P. M. 



1887, 



29 34 29-36 2936 



29-33 



29-31 



29-31 29-31 29*30 



1891, 



29*32 29-32 2933 



29-36 



29-37 



29-33 29-31 29*30 



1893, 



29-31 2931 29-34 



29-33 



29-29 



29-27 29-26 29*25 





5. 

 8 A. M. 8 P. M. 



6. 

 8 A. M. 



8 P. M. 



7. 

 8 A. M. 8 P. M. 



1887, 



2934 29-35 



29-31 



29-28 



29-28 29-30 



1891, 



2933 2936 



2936 



29-36 



29-35 29 35 



1893, 



29-30 2930 



2925 



29-26 



2929 29-31 



lean height of the barometer at Blue Hill for each day of the Six day period 





8 A. M. 8 P. M. 



8 A. M. 



8 P. M. 



o. 



8 A. M. 8 P. M. 



1887, 



29-34 29-32 



2929 



29*25 



29-28 29-31 



1891, 



29-34 29-33 



29-33 



29-31 



29-34 29 35 



1893, 



29-29 29-30 



29 30 



29*27 



29-29 29-31 





i. 



8 A. M. 8 P. M. 



5. 



8 A. M. 



8 P. M. 



6. 



8 A. M. 8 P. M. 



1887. 



2932 29-35 



29*32 



29-35 



29-33 2933 



189l| 



29-37 29-37 



29-32 



2930 



29-34 2936 



1893, 29-35 29-32 29-27 2926 29'27 29'26 



These results show very clearly the occurrence of two baro- 

 metric minima in each period. The minima occur on the 

 fourth and seventh day of the seven day period, and on the 

 second and fifth day of the sixth day period, corresponding 

 with the times of greatest frequency of minima previously 

 given. 



The next step in the investigation was to study the periods 

 from the synoptic weather charts of the United States. There 

 were two methods used, one the statistical, and the other the 

 study of consecutive individual storms. 



The average movement of storms in the United States is 

 about 15° of longitude a day toward the east ; and if the space 



