NATURE 



477 



THURSDAY. MARCH 26, 1SS5 



PRACTICAL PHYSICS 

 Practical Physics. By R. T. Glazebrook, M.A., F.R.S., 

 and W. N. Shaw, M.A., Demonstrators at the Caven- 

 dish Laboratory, Cambridge. (London : Longmans, 

 Green, and Co., 1885.) 



THE authors have clone a real service to all whose 

 business it is to conduct classes in a physical 

 laboratory by supplying them with a most excellent 

 guide. Not only teachers, but students, will find this 

 book invaluable. 



The authors have for some time prepared manuscript 

 notes for use in the laboratory, sufficient to enable a 

 student to make the measurements described without that 

 frequent necessity for supervision which is found when 

 verbal instruction only has been given. Since such well 

 tested notes form the main portion of this work, it is cer- 

 tain that the experiments which are described have been so 

 frequently carried out that the details given are sure to cor- 

 respond to the best arrangement in each case, and further, 

 that all possibility of an oversight has been removed. 



In many cases instruments used for the same purpose 

 are so different in d tail that the authors were met by the 

 difficulty of choosing whether to describe several forms 

 or to be content with explaining the particular instrument 

 used for each purpose at the Cavendish Laboratory. 

 They have, in adopting the latter course, found one 

 means of limiting an enormous subject. In another direc- 

 tion they have found a natural boundary — that between a 

 book of theoretical and one of practical physics. The 

 theory of the methods and instruments is not given at 

 length, except in those cases where the text-books are not 

 sufficiently explicit. -Again, the whole range of practical 

 physics is so extensive that choice had to be exercised as 

 to what experiments should be included and what un- 

 avoidably passed by. The experiments selected in each 

 subject are typical, and are such as "to enable the student 

 to make use of his practical work to obtain a clearer and 

 more real insight into the principles of the subjects ; they 

 include those which have formed for the past three years 

 the course of practical physics for the students preparing 

 for the first part of the Natural Sciences Tripos. 7 ' It 

 would be impossible to make a selection more exactly 

 suited to meet the wants of an educational laboratory. 



In the preface will be found the system employed at 

 the Cavendish Laboratory for making a set ot apparatus 

 go as far as possible with a large class. The subject is 

 divided into sections, each requiring its own instruments ; 

 sometimes one, sometimes several, experiments belong to 

 one section. When any section is assigned to a student, 

 none of the instruments belonging to it are available 

 elsewhere. The same system of division is employed in 

 the text, no less than eighty-two numbered sections being 

 the result. 



The value of the book is much enhanced by the ad- 

 dition, at the end of each section, of the results of an 

 actual experiment. These short statements are valuable 

 in many ways. In the first place, they show how to enter 

 results systematically, so that the meaning of the entry is 

 obvious. Secondly, they show the probable degree of 

 Vol. xxxi. — No. 804 



accuracy attainable, especially when more than one 

 method of making the same determination is given. 

 Thirdly, and this is perhaps the most important to the 

 teacher, the series of numbers to be found enables any- 

 one to discover the proportions and sizes of the several 

 parts of each piece of apparatus employed. An example 

 taken from p. 420 will make this clear : — 



"Experiment. — Determine the difference of potential 

 between the two ends of the given wire through which a 

 current is flowing. Enter results thus : — 



Mas of water 



Water equivalent of the calorimeter 



24-2 

 4'2 

 28-4 



M (copper deposited in voltameter)... - 222 

 Total rise of temperature for each two minutes : — 



4°'4 4°'4 4°' 2 4°'° 3 3 ' s 



T 2 4 °-S 



E = 4'j6 X 10 s = 4'j6 volts." 

 In this case there is no means of estimating the probable 

 accuracy of the result, but the data are sufficient to enable 

 any one who wishes to do so to reproduce exactly the 

 instrument employed. 



The chapter on physical arithmetic, in which errors, 

 corrections, accuracy, and the manipulation of small 

 quantities are treated, is of special value. 



The chapter on the balance is very complete. Though 

 perhaps the balance is the most important of all philo- 

 sophical instruments, it is a question whether so much 

 space as twenty pages should be devoted to it, where so 

 much that is important is necessarily excluded for want 

 of space. Students do certainly use the balance most 

 blindly, and if its theory is not explained in a satisfactory 

 manner in the text-books, this surely is the place to find 

 it. Other subjects of which the usual accounts in the 

 authors' opinions needed supplementing are measure- 

 ment of fluid pressure, thermometry, calorimetry, and 

 hygrometry. 



The chapters on electricity and magnetism are treated 

 in a different manner from the rest of the book, for what 

 reason is not apparent. The precise and quantitative 

 relations between mechanical, magnetic, and electrical 

 units are to be found in almost every modern text-book, 

 and so there would be no occasion to repeat definitions, 

 &c, if the treatment of these last chapters was the same 

 as that employed in the earlier ones. It is here perhaps 

 more than anywhere that the authors had to exercise 

 their choice of the most suitable, out of an almost endless 

 variety of experiments, any one of which might well find a 

 place. No one can find fault with the selection, yet it 

 seems a pity that not a word is said about electrometers or 

 indeed about statical electricity at all. Many will be dis- 

 appointed in finding no account of the absolute determina- 

 tion of electromotive force by any of the methods of 

 induction. The only method given depends on the 

 measurement of the hsat generated by a current, which 

 of course involves a knowledge of the value of J, the 

 mechanical equivalent of heat. This is the more to be 

 regretted, as instructions for determining experimentally 

 the value of J are not to be found in the chapter on heat. 

 It is to be hoped that in another edition a few pages 

 will be devoted to one or both of these essential measure- 

 ments. 



For a first edition the book is remarkably free from 



