Professor Forbes's Eighth Letter on Glaciers. 375 



N., there are, as in the south of France, rains of spring and 

 of autumn intermixed with those of all seasons. 



To recapitulate, the band of droughts and of deserts is only 

 indicated in this portion of the northern hemisphere by the sta- 

 tions of Lower California, leaving which, it is, first of all, ef- 

 faced by the junction of the estival rains with the hivernal 

 rains, and is afterwards completely obliterated on the coasts 

 of the Gulf of Mexico ; but it suffices for shewing the regu- 

 larity of the law, that it is distinctly indicated at some points, 

 and we ought only to regard the disturbances as resulting from 

 the influence of the configuration and the relief of the surface, 

 the effect of which we also endeavour to appreciate. 



The southern portion of the New World exhibits other ex- 

 amples of the same circumstances, 



( To he continued.) 



Eighth Letter on Glaciers, Addressed to Professor Jameson 

 by Professor Forbes. 



Experiments on the Plasticity of Glacier Ice. 



Geneva, 30«A Auigmt 1844. 

 My Dear Sir, — The theory of glaciers has now reached 

 that point when it can only receive some material addition by 

 the multiplication of accurate measurements ; and these mea- 

 surements must be conducted in the manner which will best 

 discriminate between rival hypotheses, and, if possible, yield 

 direct instead of indirect proofs of each fundamental fact 

 assumed. In my former letters I have insisted sufficiently 

 upon the importance of the results which a system of nice 

 measurement has introduced into this branch of science, and 

 their value to the theorist who afterwards wishes to put nu- 

 merical for unknown quantities in his investigations ; I also 

 shewed that there is a continuity and approximate constancy 

 in the motions of glaciers, which permits us to obtain, with 

 certain precautions, in a few days, better results than any one 

 had previously acquired during the lapse of months or years. 

 I have now to announce to you that I have pushed these mea- 



