108 British Association for the Advancement of Science, 



steel to the tube, at the point at which the expansion of the tube 

 is the same as that of the rod ; so that, as the steel rod expands 

 downwards, and is lengthened by heat, the zinc tube expands up- 

 wards in the same degree ; and therefore, if the lengths of the 

 rod and the tube be rightly proportioned, the pendulum may be 

 regarded as of invariable length. Some other additional appara- 

 tus is devised in order to the greater perfection of this pendulum, 

 but the description is too long for this place. 



The Secretary read the report of the Committee, consisting 

 of Sir J. Herschel, Mr. Whewell, Mr. Peacock, and Prof Lloyd, 

 appointed to represent to government the resolutions adopted by 

 the Association in August,- 1838, recommending that Magiteiic Ob- 

 servatories be established in various parts of the British dominions, 

 and that a naval expedition be fitted out for the purpose of de- 

 termining, by observations, the magnetic direction and intensity, 

 in high southern latitudes, between the meridians of New Hol- 

 land and Cape Horn. It is well known that this application was 

 eminently successful ; that the Antarctic Expedition sailed in 

 the summer of 1839, and that efficient measures liave been ta- 

 ken to secure a magnificent system of magnetic observations. 



Prof Powell made a communication on certain points in the 

 wave-theory as connected with Elliptic polarizoiion. Its object 

 was to set forth a general statement of some material conditions 

 which involve in a common relation the theory of dispersion, of 

 the wave surface and of elliptic polarization. 



Mr. E. Hodgkinson gave an account of experiments made by 

 order of the Association on the temperature of the Earth in the 

 deep mines of Lancashire and Cheshire. Satisfactory results 

 appear not to have been yet fully attained, and the experiments 

 are to be continued. 



Prof Forbes submitted a report of observations made by order 

 of the Association on the temperature of the Earth at different 

 depths near Edinburgh. These observations were commenced 

 in February, 1837, and have been regularly continued since. 

 The object was to ascertain the conducting power for heat, of 

 different soils, and the measure of the sun's influence at different 

 depths under similar external circumstances. At each station 

 four thermometers were sunk to the depths of 3, 6, 12, and 24 

 French feet respectively, the tubes of each being carried above 

 the surface so as to be conveniently exposed side by side. The 



