252 ON UNDERGROUND TEMPERATURE. 



as well as from its greater original magnitude, the annual simple har- 

 monic wave becomes more and more predominant as we descend, and 

 the curve of temperature for the year approaches more and more nearly 

 to the form of a simple harmonic curve, or curve of sines. 



" Observations taken at three stations in or near Edinburgh, and at 

 Greenwich Observatory, have been reduced in accordance with the above 

 principles, the result being in every case to show a satisfactory agree- 

 ment between theory and practice ; and the values of the thermal co- 

 efficient - thus obtained for these four stations, have furnished the basis 

 k 



of the most reliable calculations yet made regarding the earth's age as 

 a habitable globe. For the three Edinburgh stations the value of c 

 (which is the product of specific heat by specific gravity) was also de- 

 termined by laboratory experiments conducted by Eegnault, and hence 

 the conductivity, fc, was found by computation. 



" The following is a sample of the temperatures observed at Greenwich 

 at the depths of 1 inch, 12.8 feet, and 25.6 feet. The warmest and coldest 

 calendar months had the following mean temperatures : 



Warmest. Mean temp. Coldest. Mean temp. 



1 inch July 65. 9 January 40. 5 



3 feet August 62. 5 February 41. 1 



12. 8 feet September 55. 5 April 46. 1 



25. 6 feet November 52. June 48. 6 



" The mean temperature at a depth of 10, 20, or 30 feet does not differ 

 much from the mean temperature at the surface. A slight increase is, 

 however, usually observable even at these small depths ; and, when we 

 penetrate to the depth of several hundred feet, we find the temperature 

 higher by several degrees than the mean temperature of the surface. 

 In fact, the deeper we go the higher is the temperature which we find. 



"Attempts were formerly made to explain away this phenomenon, the 

 high temperatures observed in deep mines being ascribed to the pres- 

 ence of the men working in tliem, assisted in some cases by the slow 

 combustion of j)yrites ; but the fact of a steady increase downward, at 

 a rate which is not exactly uniform, but varies from about l'^ Fahr. in 

 100 feet to 1° Fahr. in 40 feet, has now been placed beyond all question." 



Theoretical investigations will be found in Fourier's Theorie ana- 

 lytique de la chaleur, Paris, 1822 ; in Poissou's Traite mathematique de 

 la chaleur, Paris, 1835. See also various papers by Quetelet in the Mem. 

 de I'Acad. roy. de Bruxelles ; also, Piazzi Smyth in Astronomical Ob- 

 servations at the Royal Observatory at Edinburgh ; Forbes and his 

 own Observations, vol. xi, for 1849-'54, and vol. xiii, for 1860-'70. A fair 

 statement of the subject is found in Schmid's Meteorology, Leipsic, 

 18G0. See, also, J. D. Forbes's " Experiments on the Temperature of the 

 Earth," in Trans. E. S. E., 1846 ; Sir W. Thomson " On the Reduction of 

 Observationsof Underground Temperature,"iu Trans. R. S. E., 1860; "On 

 the Age of the Sun's Heat," in Macmillan's Magazine, March, 1862 ; " On 



