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THE POPULAR SCIENCE MONTHLY. 



far that has brought the theory of the reduc- 

 tion of psychological facts to the finest point. 

 But with respect to the substance of method 

 and details of analysis he has, in common with 

 Mill and Bain in fact, with all the school that 

 which constitutes the organism of English psy- 

 chology and gives it a physiognomy of its own 

 in contemporary history." 



Elements of Dynamic. An Introduction 

 to the Study of Motion and Rest in 

 Solid and Fluid Bodies. By W. K. 

 Clifford, F. R. S. Part I., Kine- 

 matic. New York: Macmillan & Co. 

 Pp. 221. Price, $2.50. 



This little book plunges into depths 

 where only mathematicians can follow ; but 

 its opening sentences are so characteristic 

 of the author's clearness of perception and 

 statement, that, as they involve no formula, 

 and are withal instructive, we quote them: 



"Just as geometry teaches us about the sizes 

 and shapes and. distances of bodies, and about 

 the relations which hold good between them, 

 bo dynamic teaches us about the changes which 

 take place in those distances, sizes, and shapes 

 (which changes are called motions), the relations 

 which hold good between different motions, and 

 the circumstances under which motions take 

 place. 



"Motions are generally very complicated. 

 To fix the ideas, consider the case of a man 

 Bitting in one corner of a railway-carriage, who 

 gets up and moves to the opposite corner. He 

 has gone from one place to another; he has 

 turned round; he has continually changed in 

 shape, and many of his muscles have changed 

 in size during the process. 



"To avoid this complication we deal with 

 the simplest motions first, and gradually go on 

 to consider the more complex ones. In the first 

 place, we postpone the consideration of changes 

 in size and shape by treating only of those mo- 

 tions in which there are no such changes. A 

 body which does not change its size or shape 

 during the time considered is called a rigid 

 body. 



" The motion of rigid bodies is of two kinds ; 

 change of place, or translation, and change of 

 direction or aspect, which is called rotation. In 

 a motion of pure translation, every straight line 

 in the body remains parallel to its original posi- 

 tion ; for, if it did not, it would turn round, and 

 there would be a motion of rotation mixed up 

 with the motion of translation. By a straight 

 line in the body we do not mean merely a straight 

 line indicated by the shape or marked upon the 

 surface of the body ; thus, if a box have a move- 

 ment of translation, not only will its edges re- 

 main parallel to their original positions, but the 

 same will be true of every straight line which 

 we can conceive to be drawn, joining any two 

 points of the box. 



" When a body has a motion of translation, 

 it is found, that every point of it moves in the 



same ; bo that to describe the motion of the 

 whole body it is sufficient to describe that of 

 one point. When a body is so small that there 

 is no need to take account of the differences in 

 position and motion of its different parts, the 

 body is called a particle. Thus the only motion 

 of a particle that we take account of is the mo- 

 tion of translation of any point in it. 



"A motion of translation mixed up with a 

 motion of rotation is like that of a corkscrew 

 entering into a cork, and is called a twist. 



"Bodies which change their size or shape 

 are called elastic bodies. Changes in size or 

 shape are called strains. 



"The science which teaches how to describe 

 motion accurately and how to compound dif- 

 ferent motions together is called kinematic.'''' 



The volume is a college text-book, and 

 the genius and position of its author are a 

 sufficient guarantee of its originality and ex- 

 cellence. Prof. Clifford has broken down 

 so sadly in health that he has been com- 

 pelled to suspend work at University Col- 

 lege, in London, and leave England for the 

 more genial climate of Southern Europe. 

 His work on the "Fundamental Ideas of 

 Mathematics and Physics explained to the 

 Non-Mathematical," with which he has been 

 long occupied for the " International Scien- 

 tific Series," is well advanced, and it is to 

 be hoped that he will recover his health to 

 complete it, and carry out the other impor- 

 tant intellectual projects of which his teem- 

 ing head is full. 



Physical Technics. By Dr. J. Frick. 



Translated by J. D. Easter, Ph. D. 



With 797 Illustrations. Philadelphia: 



Lippincott. Pp. 467. Price, $2.50. 



Teachers of physical science and stu- 

 dents who are without the aid of competent 

 instructors and good laboratory apparatus 

 will find this a very useful manual. The 

 volume, in the first place, contains a great 

 deal of valuable, practical instruction for 

 making experiments in physics. The ma- 

 nipulation of apparatus, the construction of 

 apparatus at the least possible cost, the 

 points to be considered in the purchase of 

 instruments, on these and many other like 

 topics the author supplies a good deal of 

 common-sense information, gathered in the 

 course of his own experience as a teacher 

 of science, and selected from technical 

 manuals. Having aided the student in 

 choosing his apparatus or constructing it, 

 and given him some insight into the secrets 

 of physical manipulation, the author, in the 



