5i8 



NATURE 



\Oct. i8, 1877 



given a sketch of the groups or natural orders in which 

 Prof. Huxley arranges the Invertcbrata, the classifica- 

 tion of which is surrounded with so many difficulties, that 

 scarcely any two writers on classification adopt the same 

 taxonomic system. Prof. Huxley does not look upon 

 the arrangement he has adopted as more than temporary, 

 as our knowledge of the anatomy and development of the 

 Invertebrata is increasing with such prodigious rapidity 

 that the views of taxonomists in regard to the proper 

 manner of expressing that knowledge by classification 

 are undergoing, and for some time to come are likely 

 to undergo, incessant modifications. 



We heartily commend this book to all students of Com- 

 parative Anatomy. 



EVERETT'S " TEXT-BOOK OF PHYSICS" 

 Elementary Text-Book of Physics, By J. D. Everett, 

 M.A., D.C.L., F.R.S.E., Professor of Natural Philo- 

 sophy in the Queen's College, Belfast. (Glasgow : 

 Blackie, 1877.) 



IN the preface to this book the author says : " It is 

 primarily intended as a text-book for elementary 

 classes of Physics. It aims at presenting, in brief spa'ce, 

 those portions of theoretical physics which are most 

 essential as a foundation for subsequent advances, while 

 at the same time most fitted for exercising the learner in 

 logical and consecutive thought. It does not give minute 

 directions for manipulation, but, avoiding details as much 

 as possible, presents a connected outline of the main points 

 of theory. . . . The aim must be not so much to teach 

 them [the bulk of the boys in our public schools] many 

 facts, as to teach them rightly to connect a few great 

 facts together. . . . The book is not intended to super- 

 sede oral instruction, but rather to create a demand 

 for amplification and illustration such as the teacher 

 will supply." 



Judged from this point of view the text-book must 

 receive almost unqualified praise. The different divisions 

 of the book treat respectively of the subjects, dynamics 

 (in its modern acceptation), hydrostatics, heat, liglit, 

 sound, and electricity including magnetism, and in each 

 division the author explains the leading facts in clear, 

 concise, and accurate language. What mathematics is 

 introduced is of the simplest possible kind, and need not 

 prevent the veriest tyro in geometry and algebra, provided 

 he is possessed of ordinary intelligence, from reading and 

 understanding the book from beginning to end. The 

 definitions are, as a rule, very exact, and the explanation 

 of the units, as might have been expected from the author 

 of the pamphlet, the " centimetre-gramme-second system 

 of units" is singularly precise. Add to this that the 

 diagrams are numerous, and, which is of rare occurrence 

 in an English text-book, of unusual excellence, and that 

 each division is followed by a collection of examples 

 (except the last, which apparently has not been thought 

 wort'iy of the honour) at once good and easy, and enough 

 has been said to show that the text-book is one with many 

 merits. 



It has its demerits too, and if we dwell longer on them 

 it is only in the hope that a truly excellent manual may 

 be rendered still more excellent in a second edition. Why 

 does the author make not the slightest mention of 



Newton's laws of motion, although all the statements 

 made in them are asserted, but in such a casual off-hand 

 sort of way that the student wonders what is the evidence 

 for such important statements .■' For example, in Art. g 

 it is said " If a body with a movement of translation 

 (unaccompanied by rotation) is acted on either by no 

 forces or by balancing forces, it continues to move with 

 uniform velocity in a straight course." This assertion is 

 introduced by no explanation, neither is it followed by 

 any remark or illustration. The same thing may be said, 

 and even more forcibly, of the treatment which the 

 second law receives. This fault, of making assertions 

 without any explanation or shadow of proof, is rather too 

 apparent throughout the book, as a (av/ instances will 

 show. The examples are taken from pages that are all 

 near together. At p. 147, line 4, we read, " the last image 

 consists of two coincident images, as has already been 

 shown [for ' shown ' read ' asserted '] to be the case when 

 the angle is a right angle." Again, at p. 150, line 3, it is 

 said, " The angles of incidence and refraction increase 

 together and the deviation increases with them." At 

 p. 153, after a description of Airy's simple and beautiful 

 apparatus for illustrating refraction, it is added "abc 

 will be the path of a ray, and a stud at c will appear in 

 the same line with studs at A and B." At p. 158, line 9 

 from bottom, we find — " If the eye is moved with uniform 

 velocity from one side of the normal to the other (in one 

 straight line), the image moves with a velocity continually 

 diminishing till the normal is reached, becoming zero 

 at the normal, and then again gradually increasing. 

 This is a general property of geometrical images, 



whether formed by refraction or reflection " 



At p. 161, line i : — " .... The rays reflected from 

 the outer portions of the mirror will fall sensibly short of 

 the middle point of oc. If the point of incidence be 

 supposed to travel with uniform velocity along the arc 

 M O from M to N, the intersection of the reflected ray with 

 O C will move towards f with velocity gradually diminish- 

 ing to zero." Once more, on p. 163, last line, and p. 164, 

 first three lines, we read : — " .... It can be shown that 

 if the angular aperture be small all the reflected rays will 

 meet sensibly in one point, P.'' It can also be shown 

 that— 



O P ~ O P OF 



Although it is not to be expected that any other seven- 

 teen pages will supply [as many examples as these, still 

 such futults do occur throughout the book. We do not 

 object to statements heralded by the words " It can be 

 shown," as the student is at once put on his guard, and 

 virtually referred to other sources for the proof, if he 

 wishes it ; but most frequently he is left either to take 

 the statement on trust, or, in the words of the preface, 

 "to exercise his mind in logical and consecutive thought." 

 Only we think that in the latter case the text-book can 

 hardly be called one " for elementary classes." 



From inaccuracies the book is wonderfully free, but 

 there is surely one, and not a small one, about harmonics 

 at the bottom of p. 241. The author asserts that the 

 origin of harmonics depends on two very different causes. 

 Sometimes it is found in the different modes of free 

 vibration of the body which emits the sound (and this is 

 especially the case with the sounds of stringed instru- 



