Makch 14, 1913] 



SCIENCE 



415 



Through each of the five stages — Pleistocene, 

 Pliocene, Miocene, Oligocene and Eocene — of 

 the uppermost eras of geological history we 

 can trace a more or less complete gradation 

 from the horses of the present day to primi- 

 tive, many-toed animals, scarcely larger than 

 foxes, and presenting few of the features 

 which render the horse and its relatives such 

 a remarkable group. Some idea of the im- 

 mense lapse of time which has taken place 

 during the slow evolution of the Eocene 

 Hyracotherium into the modern Equus has 

 been thus expressed by Professor H. F. Os- 

 born, whom Lydekker quotes : 



The Eocky Mountains, it is true, began their 

 elevation during the close of the Age of Reptiles; 

 they had only attained a height of four or five 

 thousand feet when the Age of Mammals com- 

 menced; they continued to rise during the entire 

 period. But consider the map of Europe and 

 Asia at the beginning of Eocene time and realize 

 that the great mountain systems of the Pyrenees, 

 the Alps, and the Himalayas were still unborn, 

 level surfaces in fact, partly washed by the sea. 

 . . . The birth of the Pyrenees was at the begin- 

 ning of the Oligocene. At this time Switzerland 

 was still a comparatively level plain, and not until 

 the close of the Oligocene did the mighty system 

 of the Swiss Alps begin to rise. Central Asia 

 was even yet a plain and upland, and only during 

 the Miocene did the Himalayas, the noblest exist- 

 ing mountain chain, begin to rise to their present 

 fellowship with the sky. In North America, again, 

 since the close of the Eocene the region of the 

 present Grand Canon of the Colorado has been 

 elevated 11,000 feet and the river has carved its 

 mighty caiion through the rock to its present 

 maximum depth of 6,500 feet. 



Those who have been impressed with a sense of 

 the antiquity of these wonders of the world, and 

 will imagine the vast changes in the history of 

 continental geography and continental life which 

 were involved, will be ready to concede that the 

 Age of Mammals alone represents an almost in- 

 conceivable period of time. 



EiCHAED SwANN Lull 



Yale University 



Electricity and Magnetism for Advanced 

 Students. By Sydney 6. Starling. Long- 

 mans, Green & Co. 1912. 583 pages, with 

 452 figures. 



This book is the outcome of a number of 

 years' experience in teaching the subject to sen- 

 ior students in an English municipal technical 

 school, and it is a good book. To quote from 

 the preface, it aims " to give such students an 

 adequate knowledge of the present state of the 

 subject, with due reference to the historical 

 sequence of its development, and to the effect 

 of modern research upon it." Its seventeen 

 chapters are devoted to magnetism, terrestrial 

 magnetism, the electric current (2), electro- 

 statics (2), electrolysis, thermoelectricity, 

 electromagnetics, magnetic properties of ma- 

 terials, varying currents, alternating currents, 

 units, electromagnetic radiation, conduction 

 in gases, radioactivity, and electrons. Instru- 

 ments and methods of measurement receive a 

 great deal of consideration. Each chapter is 

 provided with a number of examples, mostly 

 taken from London B.Sc. and B. of E. papers. 



The book follows for the most part conven- 

 tional lines. Its descriptive matter is clear 

 and full and usually correct. Its mathemat- 

 ical demonstrations are ordinarily sufficiently 

 direct and simple, though it seems to the re- 

 viewer that some of them might have been 

 dispensed with and that others, e. g., those 

 pertaining to the Wheatstone and Thomson 

 bridges, would profit by simplification. The 

 calculus is freely used throughout. 



As to matters of fact the book is fairly up 

 to date. In many cases, however, important 

 recent contributions receive no mention — such 

 as the use of the methods of electromagnetic 

 induction in terrestrial magnetism, the precise 

 work of Rosa and Dorsey on the ratio of the 

 unit charges, and the brilliant work of Lan- 

 gevin, Weiss and others in the domain of 

 magnetism. 



In matters pertaining to fundamental the- 

 ory the treatment is not always logical and 

 free from looseness. Thus the definitions of 

 electromotive force and potential difference 

 are unsatisfactory; resistivity is defined with- 

 out reference to the direction of the stream- 

 lines; the curl of a vector is defined as its line 

 integral around a closed path; and Gauss's 

 theorem, demonstrated for a homogeneous 

 field only, is assumed without comment to 



