432 Correspondence — E. Hill. 



the Rothstock Cirques, the Creux du Vent (Jura), with Gravarnie 

 and other cirques in the Pyrenees ? I may add that I saw in 1908 

 from a steamer two rather small but good cirques on the flank of 

 Salina (highest point 3,156 feet) in the Lipari Islands. In these 

 and other countries, this question has been always in my mind, 

 so that I cannot recede from the position which I have assumed. 



T. G. BONNEY. 



THE CAUSES OF GLACIATION. 



Sir,— In Nature of July 29, 1920, is an article by Dr. C. Gr. Abbott, 

 of the Smithsonian Astrophysical Observatory, entitled " Solar 

 Variation and the Weather ". In it he gives an account of observa- 

 tions on solar radiation. These " showed on their face a variability 

 over an extreme range of 10 per cent. . . . The sun appeared 

 to be a variable star having ... a fluctuation in the march of 

 years ... a fluctuation running its courses in a few days, weeks, 

 or months. Both are highly irregular". The article contains com- 

 parisons of observed changes in terrestrial temperatures with 

 observed changes in solar radiation. " It is very striking that the 

 solar changes produce such large and prolonged temperature 

 effects." 



In speculations on the causes of Glacial Periods the suggestion 

 has been put forward that sun-heat may have varied in past ages. 

 Apparently there is evidence that variation is occurring even now, 

 and that its variation is caj)able of producing considerable changes 

 in temperatures on the surface of the earth. 



E. Hill. 



The Rectory, Cockfield, 

 Bury St. Edmunds. 



