NA TURE 



597 



THURSDAY, OCTOBER 17, 1901 



THE ENGINEERING LABORATORY. 

 Experimental Engineering. Vol. ii. Testing and Strength 

 of Materials of Construction. By W. C. Popplewell, 

 M.Sc. Pp. viii + 404. (Manchester: Scientific Pub- 

 lishing Company, 1901.) Price lOi'. bd. net. 

 THERE is no doubt that the most striking feature in 

 the increased provision of means for the scientific 

 training of engineers during the last quarter of a century 

 has been the great development of laboratories equipped 

 for experiment and research in all branches of engineer- 

 ing. While as yet there is no general agreement as to 

 the best avenue to the engineering profession, it is uni- 

 versally admitted that such a training as is possible in a 

 well-arranged laboratory can be made a most valuable 

 adjunct to any system of instruction or apprentice- 

 ship. This development alone has created a field for a 

 series of text-books such as that of which the book 

 before us is the second volume, while the fact that it is 

 also intended to meet the needs of the large class regu- 

 larly engaged in what is known as " commercial " testing 

 serves to widen its scope, without in any way diminishing 

 its value for more strictly educational purposes. The 

 book contains a large amount of useful matter, collected 

 with much discrimination from a great variety of sources, 

 and we have little hesitation in saying that it will in very 

 great measure meet the needs to which we have referred. 

 In his introduction, Mr. Popplewell explains that he 

 has included a chapter on the mechanics of bodies under 

 test loads, not with the view of instructing the novice in 

 the fundamental parts of the subject, but simply in order 

 to explain briefly the leading principles involved, and to 

 record the formula; to be afterwards used. Such brief 

 explanations of general principles, however, are almost 

 always unsatisfactory in themselves ; it so often happens 

 that in arriving at a simple formula the important matters 

 are those which are omitted, so that the subject is pre- 

 sented to the student in a garb of spurious simplicity. 

 Again, the student, allured by the brevity of such explan- 

 ations, is strongly tempted to confine his investigation 

 of the principles to the explanatory chapter and to over- 

 look the author's disclaimer. The results of this tendency 

 are only too apparent in the bald proofs so familiar to 

 every examiner in the subject. And further, apart from 

 its expressed limitations, Mr. Popplewell's chapter of 

 theory is in itself in some respects open to criticism. 



It is, for instance, important that the measure of a 

 "strain" should be a number representing what may be 

 called the strain-ratio, so that for any elastic deformation 

 we have the simple relation 



. . stress 



strain = ; , 



modulus 



the modulus being the stress which would produce unit 



strain. We find, however, that while " strain " is correctly 



defined on p. 14, the very next page presents us with the 



unhappy equation 



stress applied _ amount of strain 



modulus ot elasticity original lengih of the prism 

 The discussion, again, of the relation between E and G, 

 the moduli of elasticity and of rigidity respectively, is 



NO. 1668, VOL. 64] 



cumbersome and might be much simplified, and although 

 it is pointed out that the result ^that G is to E as 2 is 

 to 5) depends on assuming that Poisson's ratio has the 

 value 4, which is only partially justified by experiment, 

 this result is quoted on p. 216 as having been deduced 

 in the earlier chapter from purely theoretical considera- 

 tions. Generally speaking there is a marked lack of 

 elegance about Mr. Popplewell's tnathematics ; special 

 instances of his cumbersome style may be found on 

 p. 40, and again on p. 160. 



On the other hand, he has done well to call special 

 attention to the difference between the twisting moment 

 in a bar in which, the state being entirely elastic, the stress 

 is proportional to the radius, and that in a bar in which, 

 the plastic state having been reached, the stress may be 

 regarded as uniform over the section, a distinction which 

 is. sometimes lost sight of in reducing and comparing the 

 results of observation. 



The second chapter, a long one, is devoted to the de- 

 scription of testing machines, and the comparison of the 

 advantages and disadvantages of the various types of 

 large machines in general use will be found of great 

 value not only by the works manager or teacher who may 

 require a machine to suit special conditions, but also by 

 the intelligent student, who is only too apt to regard the 

 particular machine, on which it may be his lot to work, 

 as embodying all the virtues, or it may be all the vices, 

 of its kind. Many of the illustrations, particularly the 

 diagrammatic views of different types of machines, are 

 very good, and for a few more of these we could well 

 spare the half-dozen or so of photographs, which have 

 in most cases been taken in a bad light and have all 

 suffered in reproduction. It is a pity to burden a text- 

 book with general views which can be found, reproduced 

 in much better style, in the makers' catalogues. 



It were vain to attempt anything like an exhaustive 

 description of the various measuring appliances now in 

 use, as it seems to be a point of honour for each labora- 

 tory to design a special type, but Mr. Popplewell gives a 

 short and sufficient account of the more familiar extenso- 

 meters and gauges. More perhaps might have been 

 said of optical methods of measurement, particularly of 

 appliances depending on the principle of the optical 

 lever, which is now largely used in many laboratories, or 

 embodying a delicate spirit level, of which one instance 

 only, Prof. Unwin's extensometer, is described. 



Special chapters are devoted to descriptions of the 

 methods of carrying out various tests in tension, com- 

 pression, bending and torsion, and of recording and 

 reducing the results, and these chapters are well done. 

 In the account of bending tests we notice no reference 

 to the bending of long rods of small section. Even if 

 these tests are not of importance from the commercial 

 point of view, they are of value in the laboratory from 

 the simplicity of the apparatus and the ease with which 

 the properties of different materials can be compared, 

 and from the fact that elastic displacements large enough 

 to be accurately measured with micrometer eyepieces can 

 be used. 



Several torsion machines are described, and the diffi- 

 culty of obtaining a pure twisting couple is pointed out. 

 It is the case, however, that in many of the torsional 

 machines in ordinary use the method of measuring the 



C C 



