(LIBRARY 



PREFACE. 



The object primarily in view when the present experiments were 

 begun was a continuous record of the nucleation of the atmosphere in 

 a locality relatively free from the habitations of man, and therefore free 

 from nucleations of local and artificial origin. In other words, it was 

 to be determined whether below the fog-limit of dust- free air, i. e., below 

 the least exhaustion at which filtered air condenses without foreign 

 nuclei, the atmosphere contains any nucleation whatever beyond that 

 introduced from terrestrial sources and coming chiefly from the origin- 

 ally ionized products of combustion. An investigation of this kind 

 seemed well worth while, after it had been shown 1 that the nucleation of 

 the atmosphere, even above cities, obeys certain clear-cut laws, showing 

 a marked tendency to reach an enormously developed and sharp maxi- 

 mum in December and a flat but very low minimum in June. The 

 former at least does not in general coincide with the period of maxi- 

 mum cold, and the possibility that some effect from without was super- 

 imposed on the local effect seemed sufficiently probable to warrant 

 special inquiry. This was carried out as detailed in Chapters IV and V 

 of the present memoir, in two series of observations, made with similar 

 apparatus, simultaneously at Providence and at Block Island. The two 

 stations, lying about 70 kilometers apart, pass through practically the 

 same meteorological variations of wind and weather; while Block 

 Island, surrounded by a body of water whose smallest radius is nearly 

 20 kilometers from the center of the island, while one-half of it fronts 

 the ocean, is in the winter at least nearly free from local effect. Leav- 

 ing the detailed discussion of the results to the chapters specified, it is 

 noteworthy that the average monthly nucleations at both points of 

 observation show the same law of change, though the actual fluctuation 

 at Providence is naturally less salient. The data found at each station 

 prove that the tendency to pass through maxima in December, observed 

 at Providence in 1902-03 and 1903-04, has again unmistakably asserted 

 itself. In addition to this, however, the observations at both stations 

 developed a new and surprisingly pronounced maximum in February as 

 the chief feature in the nucleations of the last winter. Predominating 

 in each of the series of results over the earlier maximum, and holding 



1 Barus : Smithsonian Contributions, Vol. XXXIV, 1905. 



iii 



