Geology. 303 



instrument of such boundless utility that the author compares it 

 to some such fundamental device as the wheel or the lever in 

 mechanics. Chapter VI is devoted to a description of the charac- 

 teristics of the vacuum tube and is followed by a chapter each, 

 occupied with a discussion of the triode, i. e. the three electrode 

 tube, as amplifier, as rectifier, and as oscillation generator. Chap- 

 ter X discusses the use in receiving circuits and amplifiers of the 

 ''retroactive" principle by which is meant the way in which 

 the stimulation of the grid by an oscillatory current brings about 

 an introduction of energy into the oscillatory circuit from the 

 plate battery. Chapter XI gives a good account of wireless 

 telephony. The final chapter is a miscellany touching on such 

 topics as antennae, direction finding, and interference from 

 atmospherics. 



It should be remarked that the author deals almost exclusively 

 with the British practice and, as was intimated above, the book 

 for the serious student will be chiefly valuable as supplementary 

 reading to other treatises giving interesting sidelights on prin- 

 ciples differently enunciated elsewhere. 



It seems possible to inundate any discussion of radio-commu- 

 nication with a flood of mathematics which does not always leave 

 the topics any clearer than at the beginning. The author is not 

 open to this criticism for he strives to bring to light some impor- 

 tant fact from his analytical expressions and frequently com- 

 pares different arrangements by means of numerical results. 

 The typography of the book is most pleasing and in addition 

 to the 119 figures in the text twenty-four half tone illustrations 

 from photographs are introduced to show actual apparatus and 

 installations. One erratum was noticed. The reference in the 

 second equation of Chapter VII should be to p. 96. f. e. b. 



II. Geology. 



1. The White River Badlands; by Cleophas C. O'Harra. 

 South Dakota School of Mines, Bull. 13, 181 pp., 96 pis., 75 text 

 figs., 1920. — The president of the South Dakota School of Mines 

 here presents, in more popular form, a much improved re-publi- 

 cation of his ''Badland formations of the Black Hills region" of 

 1910, noticed in this Journal for March, 1911, p. 237. The new 

 edition opens with a glowing pen picture by one of our pioneer 

 geologists, the great John Strong Newberry, who contrasts the 

 present scenery and life of the great "West with that of the Ter- 

 tiary. The South Dakota badlands are wonder places for fan- 

 tastic scenery, for visible stratigraphy of fresh-water formations, 

 and above all, as a vast cemetery of antediluvian mammals which 

 have been made known to the scientific fraternity by Hayden, 

 Leidy, Marsh, Hatcher, Scott, and Osborn. The book teems with 



Am. Jour. Sci. — Fifth Series, Vol. II. No. 11. — November, 1921. 

 21 



