Jan. II, 1877J 



NATURE 



241 



The instrument employed for measuring the temperatures was 

 Ihe earth-thermometer of Magnus, which gives its indications by 

 the overflowing of mercury, which takes place when the instru- 

 ment is exposed to a higher temperature than that at which it 

 was set. To take the reading, it is immersed in water a little 

 colder than the temperature to be measured ; the temperature of 

 this water is noted by means of a normal thermometer, and at the 

 same time the number of degrees that are empty in the earth- 

 thermometer is noted. From these data the maximum tempe- 

 rature to which the instrument has been exposed can be deduced, 

 subject to a correction for pressure, which is not very large, 

 because the same pressure acts upon the interior as upon the 

 exterior of the thermometer. 



In the following resumi (as in the original paper) temperatures 

 are expressed in the Reaumur scale, and depths in Rhenish feet, 

 the Rhenish foot being 1*029722 English foot. 



Observations were first taken at intervals not exceeding lOO feet, 

 from the depth of 100 feet to that of 4042 feet, the temperature 

 observed at the former depth being ii"o, and at the latter 38*5 ; 

 but all these observations, though forming in themselves a smooth 

 series, were afterwards rejected, on the ground that they were 

 vitiated by circulation of water and consequent convection of 

 heat. 



It has often been supposed that though this source of error 

 may aifect the middle and iipper parts of a bore, it cannot affect 

 the bottom ; but the Sperenberg observations seem to prove that 

 no such exemption exists. When the bore had attained a depth of 

 nearly 3,390 feet, with a diameter of 12 inches 2 lines at the 

 bottom, an advance bore of only 6 inches' diam.eter was driven 

 1"]}. feet further. A thermometer was then lowered half-way 

 down this advance bore, and a plug was driven into the mouth 

 of this advance bore so as to isolate the water contained in it 

 from the rest of the water above. After twenty-eight hours the 

 plug was drawn and the thermometer showed a temperature of 

 36*6. On the following day the temperature was observed at 

 the same depth without a plug, and found to be 33*6. Another 

 ob.-ervation with the plug was then taken, the thermometer (a 

 fresh instrument) being left twenty-four hours in its position. It 

 registered 36 '5, and again, without plugging, it gave on the same 

 day 33 9. It thus appears that the effect of convection was to 

 render the temperature in the advance bore 3° R. too low. 



Apparatus was then employed for isolating any portion of a 

 bore by means of two plugs at a suituble distance apart with the 

 thermometer between them. This operation was found much 

 more difficult than that above described, but in several instances 

 it gave results which were deemed quite satisfactory ; while in 

 other instances the apparatus broke, or the plugging was found 

 imperfect. The deepest of the successful observations by this 

 method was at 2, 100 feet, and the shallowest was at 700 feet. 

 The first 444 feet of the bore was lined with iron tubes, between 

 which the water had the opportunity of circulating even when 

 the innermost tube was plugged, hence the observations taken in 

 this part were rejected. 



All the successful observations are given in the third column 

 of the following table, subject to a correction for pressure ; and, 

 for the sake of showing the error due to convection in the ordi- 

 nary mode of observing, the temperatures observed at the same 

 depths when no plugs were used, are given in the second 

 column : — 



These temperatures are not corrected for pressure, but they 

 are corrected for rise of zero in the normal thermometer ; and 

 this last circumstance explains the difference of 0-4 between the 



temperature 36"i5 here given and 36-55, which is the mean of 

 the above-mentioned observations at the depth of 3,390 feet. 



Another proof of the injurious effect of convection was ob- 

 tained by comparing the observed temperatures(without plugging) 

 in the first 400 feet of the great bore, designated Bore I., with 

 the temperatures observed at the same depths during the sinking 

 of another bore, designated Bore II., near it ; the observations 

 in this latter being always taken at the bottom. The following 

 were the results : — 



The temperature at the depth of 100 feet in the great bore 

 thus appears to have been raised about 2" R. by convection. 



The following is a table of the successful observations, cor- 

 rected for pressure : — 



Depth in Rhenish 

 feet. 



700 

 900 

 1,100 

 1,300 

 1,500 

 1,700 

 1,900 

 2,100 

 3.390 



Temperature 

 Reaumur. 



17-275 

 18 780 

 21-147 

 21-510 

 23-277 

 24-741 

 26-504 

 28-668 

 37-238 



Assuming, with Herr Dunker, the mean temperature of the 

 surface to be 7-18, which is the mean annual temperature of the 

 air at Berlin, we have the following increments of temperature 

 with depth : — 



The mean rate of increase found by comparing the tempera- 

 tures at the surface and 3,390 feet is exactly 1° Fahr. for 50 

 Rhenish or 51-5 English feet. 



The numbers in the last two columns exhibit upon the whole 

 a diminution with increase of depth ; in other words, the tempe- 

 rature increases less rapidly as we go deeper down. As regards 

 the first 700 feet, which exhibit a decidedly more rapid rate than 

 the rest, it must be remembered that nearly half of this distance 

 was in a different material from the rest of the bore, being in 

 gypsum with some anhydrite, while all the rest was in rock salt 

 Prof. Herschel has found, in recent experiments not yet pub- 

 lished, that the conductivity of rock salt is exceedingly high ; 

 and theory shows that the rates of increase, in superimposed 

 strata, should be inversely as their conductivities. "VVe may, 

 therefore, fairly attribute the rapid increase in the first 700 feet 



