1868. | the Temperature of the Earth’s Crust. 19 
of the particles of which it is composed, offers an accumulative 
obstacle to the passage of water downwards, more than balancing 
the hydrostatic pressure, and amounting at length to an absolute 
stoppage. This is a conclusion which wells in the New Red Sand- 
stone—itself a very porous rock—seem to offer, for after a certain 
depth has been reached, the increase of supply appears to take place 
in a diminishing ratio to the depth,* and we may suppose a zero 
point would ultimately be reached. 
As far, then, as our experience goes, it is very probable that at 
depths greater than 2,000 feet, no water would be found in ordinary 
coal-measure strata, and thus by sinking in this formation a prin- 
cipal source of error would be avoided. Other sources of error 
(such as those arising from differences in density, and conducting 
power in the several varieties of strata) beng of less moment, would 
disappear, or become inappreciable, on taking the mean temperature 
of very deep borings. 
After much consideration, the plan which we venture to recom- 
mend, in case of experiments being undertaken by the British Asso- 
ciation, or any other scientific society, would be, not to commence 
at the surface, but at the bottom of a coal-mine, of not less depth 
than 600 yards. 
There are several collieries, particularly in Lancashire and 
Cheshire, sufficiently deep for the purpose. It would be an easy 
matter to excavate a’ chamber in the coal and its roof, where the 
borings might be carried on. The chamber ought to be a short 
distance from the bottom of one of the shafts, and out of the way 
of mining operations. As the process of boring progressed, obsery- 
ations should be taken at every 10 yards, and at every change 
of strata from sandstone to shale, or coal. The boring might be 
carried down at least to a total depth of 1,000 yards from the 
surface, and having been completed under proper supervision, could 
not fail to give results of value to science. It is also probable that 
a proprietor of some colliery of the required depth would willingly 
afford the facilities for carrying on the experiment for the sake of 
the information he would derive regarding the minerals underlying 
the coal-seam then being worked. 
Before closing these observations, some further explanation is 
required regarding the depth of invariable temperature, or, in the 
words of Humboldt, the depth of “the invariable stratum.” The 
surface of the earth undergoes a change of temperature according 
to the season of the year. In summer the rays of the sun and the 
warmth of the atmosphere affect the surface and penetrate gradually 
downwards with decreasing intensity, while in winter the cooling 
* As may be gathered from the account of the sinkings at the Green Lane 
Well of the Liverpool Corporation Waterworks. See ‘Quarterly Journal of 
Science,’ vol. ii., p. 421. 
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