NUCLEATION AT PROVIDENCE AND BLOCK ISLAND. 121 



two cases must be marred by the snow effect, which is necessarily of 

 unequal value in the two cases, the depression being as a rule larger 

 and more striking as the antecedent nucleation is higher. On Decem- 

 ber 12 the two curves behave similarly, but not so December 3, 8, 13, 

 1 8, 21. The remarkable depression of the upper curve on December 



16 17 18 ~ 19 20 21 22, 23 24 



FIG. 96. Showing daily record for November 29 to December 27. 



24 is noteworthy. Some instances of detailed similarity (December 4, 

 9, 20, etc.) might be pointed out, but in general the curves in their 

 details are dissimilar. 



Differences in winds and temperatures, here as elsewhere, are in the 

 main influenced by the times at which observations were severally 

 made. The two sets of results show, as a whole (beginning with 

 November 27), the state of high contamination of the atmosphere of 

 Providence as compared with that of Block Island. Rarely do the 



