ON THE THERMAL CONDUCTIVITIES OF CERTAIN ROCKS. 61 



fore deficient in their recorded values by an eighth part of their assigned 

 magnitudes. While requiring this addition of an eighth part to their 

 magnitudes, the absolute resistances given as the practical results of the 

 experiments, require, to correct them for the same source of error, to be 

 diminished by a ninth part of their stated values. Examples of the 

 needful corrections which will suffice to remove the misconstructions 

 introduced by this entirely unsuspected error of reduction, are given in 

 the last three columns of the accompanying short Table of amended data. 



The values of the measures h and - in the three preceding adjoining 



c 



columns are increased, for correctness, in these new columns by an eighth 

 part ; while in the same columns the absolute resistances given in the former 

 columns are diminished by a ninth part of their values. The Committee 

 desires to submit this easy process of correction as an immediately neces- 

 sary treatment of all the experimental results of absolute thermal conduc- 

 tivities and resistances at which it has arrived, and which have hitherto 

 been published in the pages and Tables of these Reports, before the present 

 year, for their proper emendation. It will then be found by comparisons, 

 to which the Committee hopes to revert particularly in another year, that 

 a somewhat closer agreement than was exhibited in last year's Report does 

 actually exist between its corrected determinations and those sure, indu- 

 bitable data of rates of thermal conductivity in certain terrestrial rocks 

 which able and elaborate reductions of several extensive series of obser- 

 vations of underground thermometers have made known. A valuable 

 store of new materials, it may be noticed, for these last investigations was 

 furnished by the publication last year, in the volume of ' Greenwich Meteoro- 

 logical Reductions, chiefly for the years 1847-73,' of the continuous records 

 during this interval of twenty-seven years, of the deep-sunk underground 

 thermometers in the grounds of the Royal Observatory at Greenwich ; 

 only the first half of which valuable results have yet been utilised (by 

 Professor Everett) for deducing the constant of thermal conductivity of 

 the great eminence of gravel strata upon which the Observatory is 

 placed. 



