Notices respecting New Books. 211 



obtained. Were everything in its former state (and in a little 

 time I hope to put it so), I am confident that our former 

 estimate of the limit of practical sensibility, viz. 1 x 10~ u 

 ampere per division, would be found not far from the truth. 



1 can only rejoice if Prof. Threlfall, or anyone else, makes 

 an instrument of a higher sensibility. I hold, however, 

 that our instrument possesses special advantages in point 

 of astaticism, steadiness of zero, &c. Its principal disadvan- 

 tage is its long period of free vibration owing to the large 

 moment of inertia of the needle system. If, however, we were 

 to construct a new instrument, this would, be very greatly 

 diminished. j ain> Gentlemen, 



University College of N. Wales, Your obedient Servant, 



January 1890. A. GRAY. 



XXII. Notices respecting New Boohs. 



Physics of the Earth's Crust. By the Rev. Osmond Fisher, M.A., 

 F.G.S., Sfc. Second Edition, altered and enlarged. 8vo. Pages 

 i-xvi, 1-391. Macraillan and Co., London and New York. 

 1889. 

 npHIS excellent book comprises a complete and critical digest 

 ■*- of the various results of researches made by very many 

 Physicists in the nature and character of observed phenomena 

 and calculated probabilities relating to the structure and con- 

 ditions of the Crust and Interior of the Earth. Its value, how- 

 ever, is greatly increased by the Author's own observations and 

 calculations, especially as revised and augmented in this Second 

 Edition. A nearly lifelong experience in Geology has afforded the 

 Author good guidance in the application of his mathematical 

 studies, and has been a better basis for his researches than the 

 merely hypothetical data submitted generally to mathematicians 

 wishing to solve such terrestrial problems as come within the 

 limits of Physical Geology. Many diagrams and some plates are 

 given as illustrations. 



With reference to the new features of this Second Edition*, it 

 is stated in the Preface that " possible explanations of some of the 

 difficulties left unsolved in the first edition have since occurred to 

 me. Investigations have also been carried on by others, which 

 appear to strengthen and support some of the conclusions already 



arrived at ; these needed to be followed up and embodied A 



great part of the book has been rewritten ; and, while there are 

 many additions, there are some omissions. Some portions have been 

 omitted because they seem uncalled for in the present state of 

 Geological opinion ; and some because they would not have ac- 

 corded with the results arrived at in the new portions. At the 



* The First Edition was published in 1881. See Phil. Mag. ser. 5, 

 vol. xv. p. 56. 



