Notices respecting New Boohs. 437 



increased nearly 5 per cent, by compression between the dies used in 

 coining, and 1 per cent, by simple longitudinal extension. Such 

 changes of density are not great, but it would have been better, we 

 think, to have given the reader some idea of their extent. 



We would pause here for a moment to observe that though 

 Professor Unwin has wisely not confined himself to consulting only 

 the memoirs of engineers, but has also paid some little attention to 

 the experiments which physicists have made on elasticity, he might 

 have still further profited by the latter. We venture to say that a 

 closer inspection of such memoirs as those of Wertheim (Annates 

 de Chimie, vol. xii. 1844), Gr. Wiedemann (Wiedemann's Annalen, 

 1879, vol. vi. p. 485, and elsewhere), and of Herbert Tomlinson 

 (Phil. Trans. 1883, 1886), would have been not without profit. 

 The experiments of Wiedemann are especially valuable and 

 throw very considerable light on the effect of repeated stress 

 whether of bending, torsion, or extension. Tomlinson has also 

 shown that the internal friction of metals does not depend upon 

 the velocity with which deformation is taking place, whereas we 

 are told on page 47 that it does. 



"The author has had opportunity of examining nearly every 

 form of testing machine, and of using very nearly all the subsidiary 

 measuring and other apparatus here described." As might be 

 expected, therefore, the next four chapters, which deal with the 

 apparatus used in the engineering laboratory, are good, and indeed 

 could not well be better. It is impossible to read the description 

 of the 450-ton Emery Testing Machine at Watertown Arsenal, 

 U.S.A., without being strongly impressed. " This machine is pro- 

 bably the largest and most accurate testing machine in the world. 

 Before acceptance by the Board a link of hard iron, 5 inches in 

 diameter, was placed in the machine, and slowly strained in tension 

 till it broke at 722,000 lbs. Without any readjustment a horse- 

 hair was then fixed in the machine and broken at an indicated 

 stress of 1 lb." 



The last eight chapters are occupied with the results of testing. 

 Out of an enormous mass of data which has accumulated during the 

 last forty years, a very judicious selection has been made. The 

 tables of results which are given are all the more valuable in that 

 they have all been reduced to common units and are consequently 

 most easily understood and compared. 



We are sorry to learn from Professor Unwin that in most cases 

 Young's Modulus is measured by the ratio of longitudinal stress 

 and strain, when the last includes not only temporary but also per- 

 manent and subpermanent strain. It is no wonder that even the 

 careful observations of Professor Bauschinger (p. 250) show vari- 

 ations in the modulus which actually amount to 10 per cent., and 

 this too with instruments which would enable him to determine 

 the modulus within one-half per cent. According to Young, his 

 modulus of elasticity is the amount of end-pull or end-thrust re- 

 quired to produce any infinitesimal elongation or contraction. Now 

 provided a bar be loaded and unloaded a number of times with 



Phil. Mag. S. 5. Vol. 25. No. 15G. May 1888. 2 G 



