152 NUCLEATION OF THE UNCONTAMINATED ATMOSPHERE. 



113. Continued Dust contents of atmosphere at Providence and at 

 Block Island, R. I., compared. To interpret the curves in question 

 fully, i. c., to ascertain whether there may not, after all, be a costnical 

 effect, superimposed on the local effect observed, it is necessary to 

 make a series of observations at a station more remote from the habi- 

 tations of man. Measurements were therefore made at Block Island, 

 under my direction, by Mr. R. Pierce, jr., simultaneously with my own 

 observations at Providence, by the identical coronal method in ques- 

 tion. The stations are sufficiently close together to have nearly the 

 same meteorological elements as to wind and weather, but Block 

 Island lies well out at sea, and is, in the winter at least, nearly free 

 from local effect. 



The average daily nucleations for both stations are shown in figure 

 102, Chapter V, and, as was to be anticipated, those at Providence are 

 much in excess. Leaving these for discussion elsewhere, sufficient 

 may be learned from the average monthly nucleations in the two 

 places, given in figure 104, Chapter V. 



In both cases there was an evident tendency in 1904-5 to reproduce 

 the curves of 1902-3 and 1903-4, with the sharp maxima in Decem- 

 ber, and thereafter a rapid march toward the flat summer minimum. 

 In both cases, however, there is a new effect in February, which, by 

 being superimposed on the local nucleations at Providence, does not 

 appear further than as a determined departure from the curves of the 

 preceding years, but which juts out into striking prominence in the 

 observations at Block Island, where the local effect is relatively negli- 

 gible. Apart from quantity, the fluctuations of both curves are iden- 

 tical in character. 



Finally, as to causes of the usual solstitial maximum and minimum, 

 and of the accessory maximum in February of this year, they may 

 represent the diluted local effects averaged by the sweep of the winds 

 for an enormous extent of territory. But it is quite as reasonable to 

 keep one's mind open to the possibility that the February maximum, 

 at least, may represent an external invasion of the atmosphere on the 

 part of some external nuclei-producing agency. 



