48 4< Address of the General Secretaries 



has been in a great measure removed by the analysis of Cauchy 

 and others, who have considered the distances of the undula- 

 tory particles as quantities comparable to the length of a wave; 

 velocities of propagation of the different rays of the spectrum 

 are made to depend upon the length of wave which constitutes 

 a ray of a given colour, and upon certain constants proper to 

 the medium ; these constants being obtained from observations 

 on refractive indices for certain definite rays (or dark lines) 

 of the spectrum, the refrangibility of any other definite ray 

 (whose wave-length has been ascertained by examining an inter- 

 ference-spectrum) becomes known, and may be compared with 

 observation as a test of theory ; such experiments have been 

 made by Frauenhofer, Rudberg, and Professor Powell, who 

 has given a tabular view of the various results, without, how- 

 ever, instituting the comparison between theory and observa- 

 tion, which it would be desirable to extend further than has 

 yet been done. It would be important also to elucidate the 

 disturbing effect of temperature, which prevents even existing 

 observations from being rigorously comparable. 



The calculations respecting the tides, which have been pro- 

 secuted by the aid of the Association ever since its institution, 

 have been continued this year by Mr. Bunt, under the direc- 

 tions of Mr. Whewell. These calculations have now reached 

 such a point, that the mathematician, instead of being, as at the 

 beginning of this period, content with the first rude approxima- 

 tions, is now struggling to obtain the last degree of accuracy. 



The country in which we are now assembled, has always 

 been conspicuous for attention to meteorology, a branch of 

 physical science, in which the British Association, with its 

 power of combining the efforts of many observers in distant 

 quarters of the globe, may hope to be especially useful. 



In Scotland, Leslie opened a new train of inquiry, by ex- 

 amining the earth's temperature at different depths ; and his 

 successor in the University of Edinburgh, is now directing, at 

 the request of the Association, a large and complete course of 

 experiments on that interesting subject. Framed in conform- 

 ity with the plans adopted for similar objects by Arago and 

 Quetelet, these researches of Professor Forbes contain also 

 the means of determining the power of conducting heat, which 

 different sorts of rock possess; and may thus throw light on 

 some of those peculiarities in the distribution of temperature 

 at greater depths below the surface, which have become 

 known by experience, but are not explained by theory. 



In Scotland, Sir David Brewster was the first to obtain an 

 hourly meteorological journal for a series of years, and to draw 

 from that fertile source new and important deductions, which 



