180 



DISCUSSION OP THE ANNUAL FLUCTUATION 



The positions of the meteorological stations, embraced in the preceding table, 

 are shown on the accompanying chart by dots, to which the tabular number has 

 been attached. 



If we examine the variability of the respective dates, given in the columns of 

 " warmest day," " coldest day," and "days of mean temperature," we shall find the 

 latter confined to the narrowest limit; near these epochs the expression for T 

 reaches its greatest daily change and consequently fixes them with comparative 

 accuracy, whereas near the epochs of maxima and minima the daily change is least, 

 in consequence of which greater uncertainty must attach to these dates. 



The results for the 4 Arctic stations have been united into a mean for each epoch ; 

 even these means have less weight than corresponding values at any other station, 

 since they are based upon less than 5 years of observation. The epoch when the 

 mean of the year is reached, with a falling temperature, is the most constant for 

 all the stations ; its dates are comprised between October 8, at Polaris Bay, and 

 November 13, at San Francisco, both stations being of an exceptional character; 

 all the rest cluster closely around the 22d of October, which follows 30 days after 



