ON THE CONDUCTIVE POWEES OP VAEIOUS SUBSTANCES. 
823 
deeper than the Puits de Grenelle. The observed temperatures give an increase of 
about 1°Fahe. for 54 feet of depth. Unfortunately, no geological details are given 
respecting the nature of the strata through which this well has been sunk. It is merely 
stated, incidentally, that it penetrated the lower beds of the lias. Another bore is also 
referred to by Humboldt in his description of the above, in which also the temperature 
appears to have been very carefully observed by MM. De la Pive and Maecet, near 
Geneva. The depth was 225 metres, and the observed temperature gave an increase of 
1° Fahe. for about 55 feet. Here again no mention is made of the beds which were 
penetrated. 
A very deep Artesian well also exists at MondorfF, in the Grand Duchy of Luxem- 
bourg. The account of it is given by Aeago (p. 397). We have in this case the 
geological formations through which the well has penetrated. 
metres. 
Lias 54T1 
Keuper 206 ’02 
Muschelkalk 142T7 
Gres bigarre and Gres Vosgien , , . 311-46 
Old Schistose Rocks 16-24 
MTiole depth . . . 730-00 
The increase of temperature was at the rate of 1 °Fahe, for about 57 feet. 
In our own country there are two deep shafts of coal-mines which have afforded good 
opportunities of making observations of this kind. One is at Monkwearmouth near 
Sunderland. Professor Phillips found an increase there of 1° Fahe. for about 60 feet 
in depth. The mouth of this shaft must, I presume, be in the Magnesian limestone, 
which, with the subjacent formations of the depth of about 1700 feet, must have been 
penetrated by the shaft. 
There is also a coal shaft at Duckenfield, near Manchester, which is to be carried to 
the depth of upwards of 2000 feet. It has already attained the depth of more than 
1400 feet, and from the depth of 700 feet downwards, the proprietor, Mr. Astley, has 
obligingly had a series of careful observations made at my request. The rate of increase 
of temperature between the depths of 700 feet and 1330, was 1° Fahe. for about 65 
feet. The shaft penetrates through a number of beds of sandstone and shale, of which 
some are extremely hard and compact. They belong to the Millstone Grit and Coal 
formations. 
I have selected the above instances as those in which observations fully entitled to 
our confidence have been made at great depths, and therefore likely to be, as far as 
possible, free from the influences of local causes. But I may also add, that a consider- 
able number of observations made in the coal-mines of this country give the rate of 
increase as equal to about 1° Fahe. for 60 feet of depth ; and the same result has been 
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