284 O. Fisher—Physics of the Earth’s Crust. 
The first chapter is upon Underground Temperature, and 
recites those observed facts which lead to the universally 
received opinion that the earth’s interior is hot. hese can be 
stated very briefly. They are, 1st, the observed increase of 
temperature as we penetrate the strata, and 2d, volcanic phe- 
ena. These considerations are so familiar that no extended 
discussion is given to them. The main work of the chapter is 
devoted to an examination of the case presented by the arte- 
sian bering at Sperenberg in Prussia, which was believed by 
Prof. Mohr to lead to the inference that at the depth of only a 
mile the temperature ceased to augment with increasing depth. 
r. Fisher shows that these observations, though apparently 
anomalous at first sight, are probably not so in reality. H 
holds it to be a just inference that everywhere throughout the 
earth’s external shell the temperature increases at a nearly uni- 
form rate for each locality for 25 or 80 miles, below which hori- 
zon the increment becomes notably less rapid, and that below 
160 miles at most there is no noteworthy increase. This result 
flows from the application of Fourier’s theorem of the conduc- 
tion of heat and from the amplification of that theorem by Sir 
William Thomson. 
The second chapter has reference to the physical condition of 
the earth’s interior. Here the conclusions are necessarily very 
limited. As regards the distribution of density, it is satisfac- 
torily established that the mean density being about 5:5 and 
the external density about 2°65, the density of the interior 
approaches that of the metals, iron, silver, etc., and probably 
increases toward the center, but the-law of increase is wholly 
unknown. For the purposes of subsequent discussion, It 1s 
much more essential to frame some reasonable provisional 
by regarding the solidity of the nucleus as due to pressure an 
the solidity of the crust as due to cooling; while the interven- 
ing shell is nearly as hot as the nucleus and maintains the 
liquid condition because it is subject to a lighter pressure. 
e third chapter is a quantitative investigation of the densi- 
ties and pressures existing at various depths which follow from 
assuming Ist, the law adopted by Sartorius von Waltershausen, 
and 2d, the law of Laplace. 
In the fourth chapter he proceeds to some of the geological 
