180 REPORT— 1878. 



usually assumed the temperature of the surrounding rock, that is to 

 say, their reading has ceased to alter. The insulation of the quick- 

 silver prevents alterations during the drawing out and reading of the 

 thermometer. The correctness of the result is in no way prejudiced 

 by sediment from the boring which may yet remain in the hole-. The 

 pouring in of some water may even be useful in accelerating the experi- 

 ment. Wet bore-holes with standing water are, however, to be avoided, 

 because rock- temperature and water-temperature are not identical. 



" In the manner last described, at every available opportunity, that 

 is to say, when the work of the tunnel is from any cause compelled to 

 cease for a few days, rock-temperature observations are now instituted in 

 bore-holes ready to our hand. The observations are simple, give exact 

 results if taken with proper precaution and sufficient duration of the 

 experiment, and cause no further expense, since the thermometers, being 

 sunk in the rock, are secured against wanton injury, and there are always 

 bore-holes available." 



Dr. Stapff further states by letter that, the two original thermometers 

 supplied by Negretti and Zambra having been broken, he has had others 

 made, in which he has introduced the improvement of hermetically seal- 

 ing the outer glass case, instead of closing it with a waxed cork, which 

 gradually admitted moisture. 



In the Report for 1876 an account was given of the observations of 

 Herr Dunker in a bore about 4000 feet deep at Sperenberg, and allusion 

 was made to the undue weight which had been attached by some writers 

 to the empirical formula in which Herr Dunker sums up his observations ; 

 a formula which indicates a retarded rate of increase, and, if extended to 

 greater depths, leads to the conclusion that the temperature reaches its 

 maximum at the depth of about a mile. 



A discussion has been carried on in Germany on this subject,* chiefly 

 in the ' Neues Jahrbuch fiir Mineralogie,'&e, and the best authorities seem 

 to be unanimous in rejecting the hypothesis of a retarded rate of increase 

 in the earth's surface as unwarranted, either by the Sperenberg observa- 

 tions or any others. Herr Dunker himself concurs in this opinion. Dr. 

 Stapff also, though some of his own empirical formula? indicate a retarded 

 rate of increase, writes to Professor Everett in the following terms : — 

 " As to my formulas, I beg you to remember that they are not con- 

 structed for expressing laws of Nature. They simply are made for 

 facilitating the view over a heap of figures and data of observation. And 

 genei'ally I beg you to be sure that those formulas in my mind cannot 

 express any law for the increase of warmth at greater depths than those 

 in which the tunnel observations were made. The formulas give good 

 means for eliminating empirically some of the influences of the shape of 

 surface which occur in the profile of the mountain." 



Mr. W. Galloway, one of H. M. Inspectors of Mines, has taken 

 observations in Fowler's Colliery, Pontypridd, South Wales. The shaft 

 is 846 feet deep, and the air current down it amounts to between twenty 

 and thirty thousand cubic feet per minute. 



In order to determine the normal temperature of the coal, a hole 1^- 

 inch in diameter was bored in the side of a narrow place that was being 



* See papers by Mohr, Heinrich (two papers), Dunker, and Hottenroth, in the 

 ' Neues Jahrbuch ' for 1878, 1876, and 1877, by Brauns, in the ' Zeitschrift fiir die 

 gesammten Naturwissenschaften,' 1874, p. 483, and by Hann in the ' Zeitschrift der 

 osterreichischen Gesellschaft fiir Meteorologie,' 1878, p. 17. 



