at the bottom, 116 metres, 3°. It was thus seen that, whereas the rate of increase from the surface to 60 metres in depth was 6·4°, for the remainder of the depth, 56 metres, it was only 1·8°.
The experiments made in artesian wells have given analogous results—that is, wholly irregular as regards the rate of increase of temperature. The mean of 27 observations in Vienna is, according to Spasky, 1° to 20 metres. The very accurate experiments of Magnus, in 1831 at Rudersdorf, near Berlin, on the occasion of the boring of an artesian well, yielded the same result, but at Pregny, near Geneva, Messrs. Rieve and Marcet found the depth corresponding to an increase of 1 Cent, to be 32 metres. The well was sunk 220 metres. This figure represents sufficiently exactly the average rate of increase of temperature resulting from thermometric soundings made in artesian wells. Walferdin found an increase of 1° to 31 metres in the artesian wells of the Military School at Paris, in that at St. André (Eure) and in the well of Grenelle; and many others have given figures comprised between 30 and 35 metres for the difference of level representing a difference of 1° in temperature. The temperature of the water of the Grenelle well, 548 metres deep, and of the Passy well, 570 metres deep, is 28°, while the mean temperature of Paris is 10·6. These waters, therefore, receive from the deep strata an addition to their temperature of a little more than 17°; i. e., a little more than 1° to each 32 metres of depth. The much deeper borings of Musalweek, near Minden in Prussia, 700 metres, and of Mondorf in the grand duchy of Luxemburg, 730 metres, show a difference of 1° to 30 or 31 metres.
From a comparison of the temperatures observed by Walferdin near Creuzot, at the bottom of a boring 816 metres deep, and in a neighboring well 554 metres deep, it also appears that at these depths the heat increases more rapidly than at the surface. But wells situated very near each other may give widely varying results. Thus at Naples, according to M. Mallet, in two very deep artesian wells, distant from each other 1,600 metres, the depths corresponding to 1° of additional heat were 45 and 109 metres respectively.
The observations of M. Mohr, in 1876, in a well 4,000 feet deep, pierced through a salt-rock at Speremberg, near Berlin, led this physicist to believe that the rate of progression sensibly slackens as we descend below the surface—a conclusion agreeing with Fox's deductions from observations in the English coal-mines. M. Mohr remarked that from 700 feet, where the glass marked 19·6° Cent., to 3,300 feet, where it marked 46°, the difference in temperature corresponding to a difference of 100 feet, diminished in a regular ratio, so that, continuing the sounding, beyond 5,000 feet only a barely perceptible increase could be observed. But M. A. Boué, who warmly contested M. Mohr's conclusions, has observed with reason that percolated water will frequently lower the temperature of these deep beds, and this would explain the diminution observed by M. Mohr.
