NOVEMBEK 8, 1895.] 



SGIENGE. 



631 



such subjects as plane trigonometry, variation 

 and mensuration, etc., and witli this in view I 

 have premised the booli by a simple statement 

 of the methods of measuring angles, and the 

 geometrical meanings of sine, cosine, etc., of 

 an angle with simple explanation of the other 

 operations." This might suffice to indicate the 

 nature of the book, but it seems only fair to the 

 public to give a few more specimens of the 

 author's ideas. Naturally, we look to his 

 chapters entitled ' Inertia and the Laws of 

 Motion ' and ' Energy and Work. ' 



In the former chapter, p. 83, we read, " From 

 a large number of experiments we conclude 

 that matter is incapable of changing its own 

 state. This inert or passive condition is called 

 the inertia of matter, and the law which regu- 

 lates it is called the law of inertia." On p. 86, 

 in reference to Maxwell's statement that "the 

 change in momentum of a body is numerically 

 equal to the impulse which prodvices it, and is 

 in the same direction," the author remarks that 

 ' ' this law is sometimes called the law of impulse. 

 We must be careful to distinguish between an 

 impulse and impulsive force." Notwithstand- 

 ing this cavition, he says a few lines further on, 

 "An impulse is a force which in a finite time 

 produces a definite change of momentum." In 

 the chapter on work and energy, p. 223, we are 

 told "that there are many forms of energy, such 

 as heat, light, chemical action, electricity, 

 magnetism, etc. On this account the term 

 mechanical energy is sometimes used to denote 

 kinetic and potential energy." On p. 224, in 

 explanation of a foot-poundal he tells us ' ' this 

 is sometimes called the absolute or kinetic unit 

 of force. This unit," he adds, "was given by 

 Newton, and it is probably the most accurate." 



These illustrations, which might be easily 

 multiplied to a wearisome extent, may serve 

 to show the utterly chaotic character of the 

 work in its treatment of fundamental principles. 

 The author demonstrates clearly that if he has 

 read the works of Maxwell, Thomson and Tait, 

 etc., at all, he has read them to no purpose. 



Mechanics (Dynamics). An Elementary Text-hook, 

 Theoretical and Practical, /or Colleges and 

 Schools. By E. T. Glazebeook, M. A., F. 

 R. S. Cambridge, at the University Press, 



New York,-[Macmillan & Co. 1895. Pp. 



xii. + 256. 



This little book on dynamics is one of the 

 ' Physical Series of the Cambridge Natviral Sci- 

 ence Manuals. ' It is the outgrowth of the au- 

 thor's experience in giving a practical course of 

 lectures and laboratory work in mechanics to 

 students of medicine. The result is one of the 

 best elementary books we have seen — one well 

 worth reading, in fact, by those who have 

 passed beyond the elements of the science. 

 ' ' Mechanics ' ' the author says, in his preface, 

 ' ' is too often taught as a branch of pure mathe- 

 matics. If the student can be led up to see in 

 its fundamental principles a development of the 

 consequences of measurements he has made 

 himself, his interest in his work is at once 

 aroused, he is taught to think about the physi- 

 cal meaning of the various steps he takes and 

 not merely to employ certain rules and formulae 

 in order to solve a problem." This gives the 

 key to the plan of the book, and so well is the 

 plan executed that even the dullest reader can- 

 not fail to get instru.ction if he comes to the 

 subject without erroneous preconceptions. 



The book is divided into eleven chapters, 

 which are characterized throughout by clear- 

 ness and precision of statement and aptness of 

 illustration. The first chapter deals with units 

 and methods of measurement and with the 

 terms used in mechanics. Chapters II and III 

 are devoted to kinematics, the first to velocity 

 and the second to acceleration. Chapters IV 

 and V treat of momentum and the time rate of 

 change of momentum respectively. The term 

 force, concerning which there is commonly 

 enough obscurity even Avith mechanicians, and 

 a sort of abysmal profundity with those phi- 

 losophers who are not naturalists, appears in 

 Chapter V as the name for the rate of change 

 of momentum. 



These first five chapters furnish what the 

 author considers a sufficient inductive founda- 

 tion for the science. Thenceforth he proceeds 

 by deduction chiefly. Thus, at the close of the 

 fifth chapter he says : ' ' We are now about to 

 make a fresh start and consider Dynamics as an 

 abstract science based on certain laws or axioms 

 which were first clearly enunciated by Newton 

 and are called Newton's Laws of Motion. We 



