2 



Prof. J. Prestwich. 



be and were attached to it by different writers. I was thereby led to 

 collect the scattered evidence bearing on the subject, with a view to 

 see whether it were not possible to fix upon some more definite and 

 less elastic rate. 



If the inquiry does not lead to any material alteration of the 

 averages now usually accepted, it may, at all events, serve to eliminate 

 some sources of error, and to restrict the limits within which the true 

 rate may lie. It may also serve to show how far the differences in 

 the observations in different geological areas are due to causes com- 

 mon to all, and how far they are special to each or due primarily to 

 geological structure and local causes. 



At present, owing to the wide differences in the recorded observa- 

 tions, it is very usual to take a general mean rate of 1° F. for every 

 50 to 60* feet of depth, or of 1° 0. for every 30 metres. Others take 

 even a lesser rate of increase, and everyone must have felt the want 

 of the greater certainty which can only be given by a restriction of 

 the limits of variation to more definite bounds. I had already in con- 

 nexion with work on artesian wells, and as a member of the Royal 

 Coal Commission of 1866, in connexion with the " Possible Depth of 

 Working," got together a considerable number of observations, and 

 to those I have added a considerable number of others, many of 

 them not before recorded. Although some of the earlier observa- 

 tions may seem to be of little value, I have thought it best to keep 

 the record of all in the general list (Table I) for reference in case 

 of need, as with corrected data respecting the surface temperature 

 and height, some of them may possibly hereafter prove available. A 

 list of all local observations may be useful also at any time for further 

 research which is still much needed, particularly in connexion with 

 artesian wells, which under all circumstances, appear to afford the 

 best and surest index of underground temperatures. 



I need scarcely say that I treat the subject solely from the geo- 

 logical point of view. For its physical and mathematical aspects, 

 the papers of Sir William Thomson, f Professor Everett,^ Professor 

 Lebour,§ and the Rev. 0. Fisher, || should be consulted. 



* The more rapid gradient of 1° in 50 feet has been the one adopted by some 

 physicists and geologists, but generally one of 60 feet to the degree, or even more, 

 is adopted. 



f " On the Reduction of Observations of Underground Temperature," " Trans. 

 Roy. Soc. Edin.," (1860) vol. xxii, p. 405 ; " On the Secular Cooling of the Earth," 

 ibid. (1862), vol. xxiii, p. 157. 



X " On a Method of Reducing Observations of Underground Temperature," 

 " Trans. Roy. Soc. Edin." (I860), vol. xxii, p. 429 ; " On Underground Temperature," 

 " Proc. Belfast "Nat. Hist. Soc. for 1873-4 " ; Reports of Committee of Brit. Assoc. 

 for 1868-1884. 



§ " On the Present State of our Knowledge of Underground Temperature," 

 " Trans. North of England Inst, Min. and Mechan. Eng.," vol. xxxi (1882), pp. 59, 204. 

 jj " Physics of the Earth's Crust," Chapter I. 



