20 the Temperature of the Earth’s Crust. ~ [Jan., 
influences prevail from the surface downwards, and the result is, the 
formation of an envelope or stratum of invariable temperature at a 
short and variable distance from the surface all over the globe. 
The temperature of the invariable stratum approximates to that of 
the mean annual temperature of the place, and, according to Hum- 
boldt, its depth is regulated by the latitude (increasing from the 
Equator towards the Poles), by the conducting power of the rock or 
soil, and by the amount of difference between the temperatures of 
the hottest and coldest seasons. At Greenwich, the mean tempera- 
ture is 49°5°, and that of the invariable stratum about 50°5° at a 
depth of about 50 feet from the surface. 
Tt will be evident, on reflection, that the stratum of mvariable 
temperature is the standard of departure for all measurements at 
greater depths, and this point would require a separate series of ex- 
periments for its determination at the locality selected for the deep 
borings; but for ordinary purposes it is probable 50° Fahr., at a 
depth of 50 feet, may be taken as the temperature and depth of the 
invariable stratum over the greater part of Central England.* In 
the latitude of Paris (48° 50’) the depth and temperature of the 
Caves de ’ Observatoire (86 feet and 53°30 Fahr.) are generally re- 
garded as those of the invariable stratum. 
It is scarcely necessary to point out the questions on which an 
accurate series of observations extending to great depths might 
throw light. As one illustration, we may select the subject of meta- 
morphism of rocks, which is full of difficulties requiring solution, 
such as that presented by the rocks of the highlands of Scotland, 
where we find the metamorphosed Lower Silurian rocks reposing on 
unchanged Cambrian sandstones. Again, the change of bituminous 
coal into anthracite is only as yet partially explained in such instances 
as those of South Wales and the coal-field of the Don, where the 
same beds occur in both forms at opposite sides of the field. The 
nature and the fluidity of the interior of the earth itself is also at 
this moment (and probably ever will be) a matter of controversy 
among physical philosophers,t and one of those questions on which 
possibly some light might be thrown by the proposed experiments, 
though considering the small fraction of the earth’s radius which 
comes within reach of man’s feeble operations, one cannot be very 
sanguine on this head. It is to be remembered, however, that in 
prosecuting physical researches, it is not necessary to have a definite 
end in view, except that of adding to our knowledge. No new and 
well-certified observation will be allowed to lie for ever useless. 
* See Professor Forbes’ ‘‘ Experiments on the Temperature of the Earth at 
different Depths.’”’-—‘ Trans. Royal Society of Edinburgh,’ vol. xvi., 1849. 
+ I reier more particularly to the views stated by Dr. Sterry Hunt in his lecture 
before the Royal Institution, London; and the reply thereto by Mr. D. Forbes.— 
See ‘ Geological Magazine,’ vol. iv., pp. 357 and 443. 
