ON THE THERMAL CONDUCTIVITIES OF CERTAIN ROCKS. 129 



The appearance, in the first quarter of this century (in the year 1822), 

 of Fourier's work on the laws of heat-conduction, and later on (in the 

 year 1835) the production by Poisson of a similar treatise, together with 

 publications by these and by Rudberg, Quetelet, Kuppffer, and other 

 wi'iters of distinction on the same subject in the interval between those 

 dates, directed the attention of those concerned in investigations of terres- 

 trial physics to a new method of enquiry regarding the debated question 

 of the globe's internal temperature. Between the upholders of the rival 

 theories of the earth's original igneous, or aqueous consolidation, the reality 

 of the argument of the gradual increase of temperature with depth 

 below the surface of the earth was clearly demonstrated and established 

 at about that time by the works of Cordier in Prance, and of Fox and 

 Henwood in England, on the temperature of mines in deep workings at 

 ma,ny different locahties of the globe. The evidence which this,*now 

 universally established phenomenon presents in the views of Fourier's 

 theory of a waste (presumably constant) of the globe's internal heat, has 

 pointed not only to an extremely high present, but also to a much higher 

 past internal temperature ; and the verification and extension of Fourier's 

 theory by legitimate and practicable experimental trials is a course of the 

 greatest speculative, and, without doubt, also of the greatest practical, 

 consequence and interest to geologists. It is a meritorious recollection of 

 its earliest proceedings that, among its first few years' recommendations, 

 the British Association assisted and substantially endorsed these views by 

 directing at that time the establishment in Edinburgh, under a Meteoro- 

 logical Committee, of which Professor Forbes, Professor PhUlips, Colonel 

 Sykes, Dr. Apjohn, and other distinguished men of science were most 

 efficient members, of the three series of rock- and earth-thermometers in the 

 trap-rock of the Calton Hill, the sand of the Botanical Garden, and the 

 sandstone of Craigleith Quarry, near Edinburgh, which were read and 

 recorded weekly, and finally reduced to thermal data for the five years 

 following theii' establishment, from May 1837 to May 1842, by Professor 

 Forbes.' The two last of these thermometer-sets were then abandoned, but 

 the weekly readings of that in the Royal Observatory grounds on the 

 Calton Hill in Edinburgh were continued uninterruptedly, until the recent 

 destruction of the instruments, for forty years. 



The records of five years during which observations were recorded at 

 all the three sets of thermometers by Professor Forbes, and of thirteen 

 following years, were similarly reduced to thermal data, in the year 

 1860, by Sir W. Thomson f and Professor Everett also calculated from 

 a seventeen years' period of the same observations a coefficient of con- 



' Transactions of the Boyal Society of Edinhurgh, vol. xvi. p. 219.— The co- 

 etticients of conductivity there arrived at are given, like Poisson's, in French metres 

 and the year ; for trap, sand, and sandstone, A=ll-120, 8-260, 20884 ; roquirino- for 

 reconversion to Paris-feet (the unit of depth of the thermometers) to be multiplied 

 oaaoi '/-* square of the number of Paris-feet in a metre, giving 105 38, 78'^S 

 K «■ \v .?.? '^^"°^ *^®'^® figures wth those in Paris-feet, 124-2, 78-31, .319-3 givon 

 by bir W. Ihoinson's discussion on the page quoted belo-w, it will be seen that Sir W 

 .Ihomsons citations of Forbes's values ('111-2, 82-6, 298-3') at that place, are 

 erroneous in havmg only been multiplied by 10, instead of by 9-477, for their con- 

 version ; and that the two results for sand are, in fact, almost identical, while sensible 

 differences do actually exist in the values found by Professor Forbes and Sir William 

 Thomson for trap-rock and Craigleith sandstone. The values of Forbes' data entered 

 ^f w ^u''^^" 1 able are those furnished directly by the origmal conclusions as above, 

 r^f his above-quoled Memoir. ■> a , 



-,'q^' ^°^- ^^^- P- ^'^-^^ <e£.,.(theconcluded-data.on p. 425). 

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