160 « Proceedings of the British Association. 



western coasts, and over land to the eastern, is the cause of the 

 difference in the temperature. This inference, is not however in 

 accordance with facts, as the low temperature is not proportional 

 to the distance from the western coast. Throughout this part of 

 the northern hemisphere, it is found that climate has certain rela- 

 tions to the elevation of land, not simply arising out of the ele- 

 vation of that part of the earth's surface above the general level, 

 but out of the influence which the elevation exercises on the 

 atmosphere. After an extensive discussion of facts, Mr. H. con- 

 siders himself warranted iri concluding that the great difference 

 in the winter climates of certain parts of the northern hemis- 

 phere, is attributable to elevations of land intercepting and con- 

 densing atmospheric steam, and thus rendering those parts wet 

 and warm, while cutting off" the supply from the more northern 

 parts, leaves th^m dry and cold. 



On the theoretical computation of Refractive Indices, by Prof. 

 Powell. — In the Report on Refractive Indices, which the author 

 had presented to the Association, his professed object extended only 

 to exhibiting the results of observation without any reference to 

 theory. Since that report was made, he has devoted his attention 

 to the subject of their theoretical computation, and it is the object 

 of the present communication to state very briefly the progress 

 made in it. The results in the Report on Indices are classified 

 under three heads : 1, those of Fraunhofer ; 2, those of Rudberg ,• 

 3, those derived from the loAest observations of the author, com- 

 prising many neio results, superseding former ones; and others, 

 the combined results of several sets of earlier observations com- 

 pared with later. The first series was compared with theory, 

 1, by the author in the Phil. Traris. 1835, but only by an ap- 

 proximative and tentative method ; 2, by Mr. Kelland, by a direct 

 and exact method in the Camb. Trans. Yol. 6 ; 3, for the rays 

 D and C only, by Sir W. R. Hamilton in the Phil. Mag., 3d 

 series, Vol. 8 ; 4, by M. Cauchy in the Nouv. Exerc. livr. 3-6, 

 by a most exact and elaborate process. The second series has 

 been computed only by the author, by the same approximative 

 method as the first, in th& P/w7. Trans. 1836, whence it was re- 

 printed in Poggendorff's Ajinalen. Some of the first results be- 

 longing to the third series were computed by the author, by Sir 

 W. R. Hamilton's method, in the Phil. Trans. 1837, and three 

 of the higher cases, in which discrepancies appeared, were recom- 



