78 KEPOET— 1882. 



the respective depths of 263 m. 400 m. and 600 m., show, by comparison 

 with one another and with the constant temperature in the artificial 

 caves under the Paris Observatory, a rate of increase of 1° F. in 56 or 

 57 feet (IV. 24, 25). These data would give at the depth of 100 m., or 

 328 feet, a temperature of 57°, and at the depth of 660m., or 2,165 feet, 

 a temperature of 90° ; whereas the temperatures actually observed at those 

 depths in the well at La Chapelle in October 1873, when the water had 

 been undisturbed for a year and four months, were 59°'5 and 76° (VI. 254) . 

 It thus appears probable that the upper part of the well is warmed, and 

 the lower part cooled, by convection. Further light may be expected to 

 be thrown on this point when the well reaches the springs, and the water 

 spouts above the surface, as it does at the Puits de Grenelle. A letter 

 received by the Secretary in July 1882 states that engineering difficulties 

 have prevented any deepening of "the well since the above observations, 

 but that arrangements for this purpose have now been made. 



More certain and pi'ecise information as to the effect of convection in 

 deep bores is furnished by the experiments of Herr Dunker at Speren- 

 berg (IX. 204-208). The principal bore at Sperenberg has a depth of 

 4,052 Rhenish, or 4,172 English feet, and is entirely in rock salt, with 

 the exception of the first 283 feet. Observations were first taken (with 

 a maximum thermometer on the overflow principle) at numerous depths, 

 from 100 feet to the bottom, and showed a fairly regular increase of tem- 

 perature downwards. The temperature at 700 feet was 16°"08 R., and at 

 3,390 feet 34°-l R. Plugs were then contrived which could be fixed 

 tight in the bore at any depth with the thermometer between them, or 

 could be fixed above the thermometer for observinfj at the bottom. 

 Convection was thus prevented, and a difference of one or two degrees 

 Reaumur was found in the temperatures at most of the depths ; at 700 feet 

 the temperature was now 17°'06 R., and at 3,390 feet 36°-15. We have 

 thus direct evidence that convection had made the temperature at 

 3,390 feet 2°-05 R., or 4°-6 F. too low ; and this, as Herr Dunker 

 remarks, is an under-estimate of the error, inasmuch as convection had 

 been exerting its equalising action for a long time, and its effect could 

 not be completely destroyed in the compai'atively short time that the 

 plugs were in position. Again, as regards the effect of convection on the 

 upper part of the bore, the temperature 11°'0 R. was observed at the 

 depth of 100 feet in the principal bore when no plugs were employed, 

 while a second bore only 100 feet deep in its immediate vicinity showed 

 a temperature 9°*0 R. at the bottom. This is direct evidence that the 

 water near the top of the great bore had been warmed 2° R. or 4^° F. by 

 convection. 



Suggestions for observations in filled-up bores will be found in the 

 eleventh report, but they have not yet taken a practical shape. 



D. Questions affecting deductions from Obseevations. 



1. In many instances the observations of temperature have been con- 

 fined to considerable depths, and in order to deduce the mean rate of 

 increase from the surface downwards it has been necessary to assume the 

 mean temperature of the surface. To do this cori'ectly is all the more 

 difficult, because there seems to be a sensible difference between the mean, 

 temperature of the surface and that of the air a few feet above it. 



In the third report some information on this point is given, based on 

 observations of thermometers 22 inches deep at some of the stations of 

 the Scottish Meteorological Society, and of thermometers 3 (French) feet 



