126 REPORT — 1881. 



incomplete apparatus, or to give a very satisfactory account of experi- 

 ments still in progress ; but as it is now two years since the Committee 

 was appointed, we have thought it best to give to the British Association 

 such an account as we can of our progress. 



Second Report of the Committee, consisting of Captain Abney, 

 Professor W. G-. Adams, and Professor Gr. Carey Foster, ap- 

 pointed to carry out an Investigation for the purpose of fixing 

 a Standard of White Light. 



Since the last meeting of the Association but little progress has been 

 made in the investigations. Though several series of experiments have 

 been made, no definite conclusion on the subject has been arrived at by 

 your Committee. Owing to the accidental omission to present a report to 

 the last meeting, the recommendation embodied in the communication 

 which was printed in the last annual volume could not be carried out. 

 (See 'Reports Brit. Assoc' 1880, p. 119.) 



Filial Report of a Gominittee, consisting of Professor A. S. Her- 

 SCHEL, Professor W. E. Ayrton, Professor P. M. Duncan, Professor 

 Gr. A. Lebour, Mr, J. T. Dunn, and Professor J. Perry, on 

 Experiments to determine the Thermxil Conductivities of certain 

 Rocks, shotuing especially the Geological Aspects of the Investi- 

 gation. 



In bringing to a close the series of Reports which it has submitted during 

 the past series of years since 1874, the Committee has endeavoured to 

 collect and to compare together the several exact and well-deduced 

 results from observations arrived at hitherto by various independent 

 experimenters and investigators in the subject of its inquiry, so as to 

 show at once the present position of the exjjerimental research and the 

 most essential points in which it requires further extensions and improve- 

 ments. .II,-,. 



The method pursued by Professor Everett in his work on ' Units and 

 Physical Constants,' of expressing all the well-determined data of phy- 

 sical experiments in terms of the centimetre, the gi'amme, and the second 

 as a common system of units, is adopted in forming the general list of 

 absolute and relative thermal conductivities by different observers, which 

 the Committee has met with and collected together in the simple order 

 and arrangement of a classified Table of thermal properties of rocks pre- 

 sented with this Report. Many of the data presented in the Table are 

 therefore already furnished in the uniform and well-authenticated form 

 required, in Professor Everett's work. But the result of the present 

 comparison has afforded the Committee such positive grounds of confi- 

 dence in the general accuracy of the values found by its long-continued 

 series of experiments, that it has been enabled by that means to assign 

 absolute values to the relative ones of several important lists of thermal 



