288 M. L. Cordier, Exammati<yn of recent Experiments 



are far from being so satisfactory as there was reason to expect 

 from the number of experiments that have been made, and the 

 perseverance which several observers have applied to them. We 

 are indemnified to a certain degree by the exception which is to 

 be made in favour of the experiments of the same kind, but 

 sedentary, which have been carried on for so long a time in the 

 old quarries called the Caves of the Observatory of Paris. These 

 are conclusive, and are capable of yielding a numerical and ab- 

 solute result. Their accuracy affords a compensation for the 

 small depth which they embrace. They incontestibly announce 

 a pretty rapid increase of the subterranean heat. At the level 

 of 92 feet, the mean temperature of a thermometer immersed 

 in a recipient filled with sand, and supported by a pillar, keeps 

 at 1°.8 beneath the mean external temperature. In the course 

 of a year, the variations of the thermometer do not exceed g^jd of 

 a centigrade degree. 



^ Such is, in fine, the merit of the experiments that have been 

 made upon the temperature of the air in the cavities, by means of 

 which we can penetrate into the bowels of the earth. We shall 

 now examine whether the results that have been obtained by 

 proceeding in a different manner, and especially by consulting the 

 temperature of the waters which exist in mines, present more 

 numerous or more certain resources, with reference to the object 

 which we have in view. 



2. Temperature of the Water in Mines. — Water presents 

 itself in various ways in mines. Here it issues from the rock 

 under the form of filtrations, more or less copious ; there it tra- 

 verses the bottom of the excavations in small brooks. Elsewhere 

 it is stagnant, and constitutes pools or true subterranean lakes. 



Not viewing the observations which have been made on the 

 water thus contained in mines, otherwise than as merely forming 

 a mass of approximative documents, we may yet, without hesi- 

 tation, conclude from them that there exists a notable increase 

 in the subterranean heat. In fact, the experiments were made 

 at different seasons, and the results are all higher than the mean 

 temperature of the country where they were performed. The 

 differences increase rapidly as the depth increases. Whatever 

 influence may be attributed to the summer rains, with reference 

 to the temperature of springs and filtrations, to the air during 

 warm weather, or to the lights and the presence of the work- 



