NO. 17.] 



TEMPERATURE OF THE AIR. 



491 



The most frequent period is in general that of one day, of which we 

 have from 3 to 4 per mean month. The frequency of this period is greater 

 in the summer and autumn than in the spring and winter. The frequency 

 of the other periods decreases with the length of the period, at first, up to 

 the period of 4 days, very evenly, then more slowly to the longest period of 

 6 days, the frequency or probability of which scarcely reaches O'l per month. 



