Notices respecting JVeio Boohs. 57 



been left unsolved, and some which have occurred to me not even 

 mentioned." 



The first question discussed (Chap. I.) is the underground tempe- 

 rature* ; and a result given as approximately true near the surface 

 is, in agreement with the view, we believe, now generally adopted, 

 that the law of increase of temperature is " on the whole an equable 

 one, amounting on an average to about one degree Fahrenheit for 

 every 51 degrees of descent." 



Mr. Fisher's theme is the Physics of the Earth's Crust ; for if 

 we know little comparatively of this, much less do we know of the 

 condition of the interior of the earth. Chapter II. deals specially 

 with this subject ; and in it we have an interesting discussion of the 

 views of Sir W. Thomson, Messrs. Delaunay, Hopkins, and others. 

 Is any portion at the present date liquid? and if so, how much of 

 it ? As a matter of comparative certainty, Mr. Fisher, in agree- 

 ment with other physicists, infers from the present form of our 

 globe, and from the law of variation of gravity on its surface, that 

 it was once wholly melted, and now consists of concentric sphe- 

 roidal shells, each of equal density throughout and of definite form 

 (cf. J. D. Forbes, § 256 &c, ' Progress of Mathematical and Phy- 

 sical Science,' and Col. Clarke's 'Geodesy,' cap. iv. & xiii.). We 

 give here the author's summing-up in partial opposition to Sir 

 W. Thomson's hypothesis of a rigid earth: — "It does not appear 

 necessary that the earth should be absolutely solid from the centre 

 to the surface to satisfy the requirements of great rigidity as a 

 whole. May this not be satisfied by the hypothesis of a rigid 

 nucleus nearly approaching the size of the whole globe, covered by 

 a fluid substratum of no great thickness compared to the radius, 

 upon which a crust of lesser density floats in a state of equili- 

 brium?" On the other side we may cite Prof. Green ('Nature,' 

 March 23, 1882), who considers that Mr. Fisher's work " seems 

 rather to show that the earth did not consolidate in the way sup- 

 posed by Sir W. Thomson," and that there are other equally pro- 

 bable hypotheses as to the way in which the earth may have passed 

 from a fluid to a solid state. In Chapter III. we have a discussion 

 of the internal densities and pressures — corrections of Walters- 

 hausen's theory, and fluid pressures within the earth calculated 

 from Laplace's law of density. Chapter IV. treats of the lateral 

 pressure, and examines the generally received views of the effect of 

 lateral pressure on the contortion of rocks. Chapter V. examines 

 the truth of the assumption that, because the pressure thus pro- 

 duced would be sufficient for the purpose, the work has been 

 accomplished in this way. The next Chapter is principally occupied 

 with a criticism of Sir W. Thomson's paper " On the Secular Cool- 

 ing of the Earth," and an account of Mr. Mallet's experiments. The 

 following chapter (VII.) summarizes the results of this, and con- 



* Our author commences with this discussion, because " the distribu- 

 tion of heat in the interior of the earth is one of the cardinal conditions 

 upon which all physical questions connected with.it depend." 



