464 PROCEEDINGS OF SECTION C. 



Carboniferous glacial beds to those of the Pleistocene being about as 

 10 to 1, our earth may as yet have experienced only about one-tenth 

 of the Great Ice Age, that is to say nine-tenths may yet have to be 

 experienced. 



An alternative hypothesis is that, without lowering earth tempera- 

 ture so as to shift the isotherm of G° C. from near the Antarctic Circle 

 to somewhere near the neighborhood of the anticyclone belts, some 

 causes may conspire to uniformly increase or decrease, as the case may 

 be, the rainfall and snowfall all over the world. Such a cause may 

 possibly exist in what is known as atmospheric nucleation or atmos- 

 pheric ionisation. The researches of J. J. Thompson and of C. R. 

 Wilson have shown the importance of ionisation in favoring conversion 

 of water vapor into the form of water dust ; in other words, in favoring 

 the production of fog and cloud and so facilitating precipitation. 

 Recently Professor Carl Barusf has shown that the number of the 

 nucleations in the atmosphere in the neighborhood of Providence, 

 New Jersey, varies very regularly. He shows {op. cit., p. 224) a curve 

 for mean monthly nucleations from October, 1902, to October, 1904, 

 and draws attention to the fact that the maxima and minima of nuclea- 

 tion occur during the winter and summer solstices respectively. He 

 comments that (op. cit., p. 225), " The identification of maxima with 

 the winter solstice and of minima with the summer solstice is alluring, 

 for, in these cases, the earth is, respectively, nearest and farthest from 

 the sun. At the same time the orbital velocities are, respectively, 

 greatest and least, so that the path volume of the earth would have 

 corresponding values. Both causes are qualitatively in harmony with 

 the observed results if the nucleation comes in great part from the sun. 

 Quantitatively the results are less convincing." For example, he shows 

 that the ratio of greatest and least nucleation is about 3 to 1, which is 

 enormously in excess of what it should be numerically as deduced 

 from the greatest and least values for the radius vector of the earth's 

 orbit. This and other valuations only give an aggregate valuation of 

 the order of about 6-7 per cent. In other words, a decrement of about 

 2 per cent, of distance of the sun from the earth corresponds to an 

 increment of about 100 per cent, of nucleation. While he states that 

 in some cases the rise in atmospheric nucleation can be attributed to 

 definite sources, as to sun spot disturbances, or to smoke particles 

 derived from forest fires, the main cause of the remarkable difference 

 between winter as compared with summer nucleation is at present 

 unknown. It is clear, therefore, that such an important factor as 

 nucleation, which has exercised so wide a control over rainfall and snow- 

 fall, is one which must by no means be neglected by students of the 

 glacial problem. 



In suggesting the above theories the author by no means seeks 

 to tempt away his colleagues from the narrow road of careful and 

 constant collecting of evidence which ultimately leads to the discovery 

 of fresh truths. His object in tentatively putting forward hypotheses 

 has merelv been to show how verv wide is the scope of the inquirv, 



t Smithsonian Contributions to Klnowledge, part of vol. xxxiv. 



