September 29, 1923J 



NA TURE 



467 



sets would certainly cause complaint. In describing 



the different forms of apparatus which have been used 



for measuring vibrations^ no hint is given as to the 



trustworthiness of the results. This is an important 



omission^ for in the greater number of those instruments 



the records are an imperfect catalogue of peculiarities 



of the instrument rather than of the magnitude of the 



external vibrations which they were designed to 



ieasure. All such instruments have natural periods of 



leir own, and one of the most important points in 



leir design should be to arrange that neither the 



>west nor any of the more rapid natural periods shall 



)roach those of the imposed vibrations, and since in 



)st cases the imposed vibrations are (like white light) 



ide up of a great many arbitrary disturbances, this 



not a condition which it is easy to fulfil. Many 



listaken diagnoses have been made from neglecting 



^e effects of resonance on the recording apparatus, 



id from supposing that a large recorded amplitude 



fecessarily indicates a large external vibration. 



iPerhaps the most interesting chapter is that on 



^e isolation and damping of sound, in which many 



imples are given of successes and failures in practical 



^tempts in this direction. In most of these the actual 



suits might have been anticipated. In speaking of 



^e minimum audible sound (as in reference to the least 



isible vibration) insufficient prominence is given to 



|e effect of the surrounding conditions. In an 



solute silence many experiments have shown that a 



ind, the wave amplitude of which is a twenty-five 



millionth of an inch, can be heard, but in the midst 



o£ other noises, if the amplitude of the loudest of these 



IS taken as unity, another sound with an amplitude of 



1/15 is only just audible, so that the greatest and least 



intensities which can be appreciated simultaneously 



re something like two hundred to one. 



With regard to the isolation of sound, an absolute 



barrier to the propagation of vibrations may be set up 



either by complete reflection or complete absorption, 



but when the amplitude is large and the absorption 



rapid, a gradual change may probably occur in the 



bsorbent. The secular change in the efficiency of 



•und-absorbing materials is not mentioned. 



In reference to the acoustic qualities of halls and 



'oms,.most of the experimenters whose work is quoted 



cm to consider that " good " and " bad " depend 



;i the rate at which vowel sounds and musical notes 



re damped, but it is not uncommon to find rooms 



A liich are good for music but bad for speech, and it is 



the effect of the resonance of the room on the con- 



')nants rather than on the vowels which determines 



A l\ether spoken words are clearly heard. 



Though there are many published papers on the sub- 

 iicts which (onic iiiulcr the head of " vibratimi," "\rr. 



NO. 2813, VOL. 112] 



Eason's is the only book in which any collection of their 

 results has been attempted, and notwithstanding some 

 defects (chiefly of omission), it should form a very use- 

 ful addition to the literature of the subject. A. M. 



Our Bookshelf. 



Advanced Practical Physics for Students. By B. L. 

 Worsnop and Dr. H. T. Flint. Pp. vii + 640. 

 (London : Methuen and Co., Ltd., 1923.) 21s. net. 



Teachers of experimental physics will find much that 

 is useful and suggestive in this volume. Though some 

 experiments of an elementary character have been 

 included, the work is intended for advanced students 

 who are working for a pass or honours degree. The 

 bulk and the price of the book might have been reduced 

 materially by the omission of much that is common to 

 many elementary text-books. In some cases full experi- 

 mental details are given, while in others the description 

 is insufficient to enable an ordinary student to carry out 

 the necessary manipulations. Little attention is given 

 to the degree of accuracy to be expected. 



Many recent experiments and modern forms of 

 apparatus have been described. We may mention in 

 particular the determination of the ratio of the charge 

 to the mass for an electron by means of the Zeeman 

 effect using a Lummer-Gehrcke plate, and also by Sir 

 J. J. Thomson's method. From the account given in 

 the book the student might infer that the latter method 

 is due to Braun. There is a useful chapter on the 

 quadrant electrometer (in which Wheatham should be 

 Whetham), and a section on the three-electrode valve. 



The most striking feature of the work is the stress 

 laid on the theoretical side of the subject, the aim being 

 to make the course practically independent of other 

 treatises, at least as regards immediate reference. To 

 aid this scheme an introductory chapter on the calculus 

 has been included. 



It is to be regretted that the proof-sheets were not 

 submitted to a literary critic, as there are too many 

 examples of careless or ungrammatical construction, 

 and the punctuation needs amendment in many places. 

 The wholly inadequate table headed " Units " needs 

 revision : the value for the electrochemical equivalent 

 of hydrogen has long been superseded, and to give the 

 charge on an electron as 4-7ixio~2o E.S.U. is un- 

 pardonable. 



Mechanical Testing: a Treatise in Two ]'ohi!J!cs. By 

 R. G. Batson and J. H. Hyde. Vol. 2 : Testing of 

 Prime Movers, Machines, Structures and Engineering 

 Apparatus. (The Directly-Useful Technical Series.) 

 Pp. xi + 446. (London: Chapman and Hall, Ltd., 

 1922.) 255. net. 



The first volume of this work dealt with the testing of 

 materials of construction ; the present volume concludes 

 the treatise and contains a great deal of matter which 

 will be of service to all who are interested in the testing 

 of machines and structures. The selection of a .suitable 

 dynamometer is of vital importance in the testing of 

 an engine or machine, and, roughly, one-quarter of the 

 volume is devoted to different types of this instrument. 

 This scilion includes traction (1\ nannunctcrs snrh as 



