688 



SCIENCE. 



[N. S. Vol. 1. No. 25. 



former have the advantage of being readily 

 apprehended, those of the latter of being 

 niathematicall3' exact. 



Sir Henrj^ Eoscoe deserves the thanks of 

 all workers in chemistry for having provided 

 them with an nnusuallj' interesting biogra- 

 phy of one of the founders of the science. 

 Edward H. Keisee. 



Beyn Mawe College. 



JEladicitat unci Festigheit. 'Ry C. Bach, Pro- 

 fessor in the Technical High School at 

 Stuttgart. Second Edition. Berlin, Ju- 

 lius Springer. 1894. Octavo, 432 pages 

 and xiv plates. 



In this work the author lays down the 

 -guiding principle that the student of me- 

 chanics of materials should first of all be- 

 come acquainted with tlie actvial phenom- 

 ena of stress. To this end photographic 

 illustrations are given exhibiting the de- 

 formations of bars under tension, of blocks 

 under compression, of beams and plates 

 under flexure and of shafts under torsion. 

 These illustrations are most useful and 

 show the typical changes of form in a beau- 

 tiiful manner. ISTevertheless their value is 

 probably not so great as the author as- 

 sumes, for nearly all the theories and com- 

 putations of the mechanics of materials are 

 confined to the case where the elastic 

 strength is not exceeded and where changes 

 of form are not perceptible to the eye. 



The modulus or coefficient of elasticity, 

 usually represented by the letter E, is not 

 employed in this book. Instead its recip- 

 rocal is used and called the extension co- 

 efficent, which may be defined as the 

 stretch of a bar per tinit of length due to a 

 stress of unity on each square unit of cross 

 section. There can be no doubt but that 

 the term coefficient of elasticity is a most 

 unfortunate one, as it has no relation to 

 elasticity in the ordinary sense of the word, 

 but is a measure of stiffness or rigiditJ^ 

 The improvement desii'fed would be a 



change of name rather than the inti'O- 

 duction of a new term and sj'mbol. Even 

 the author, who uses the new constant con-^ 

 sistently in all his formulas, rarely gives 

 numerical values for it, but expresses these 

 in terms of its reciprocal, which is, of 

 course, the coefficient of elasticity as uni- 

 versally employed. 



The scope of the work is that of a text- 

 book on the mechanics of materials and of 

 beams, columns and shafts, suitable for 

 technical schools -which desire to avoid ex- 

 tended mathematical discussions. The 

 usual theoretic formulas are demonstrated 

 in a neat manner, and many results of 

 tests are presented ; those on circular, el- 

 liptical and rectangular plates may in par- 

 ticular be noted as novel and valuable. 

 The subject of internal work or resilience is 

 discussed more fully than in British or 

 American books. True internal stresses 

 resulting from the change of shape are 

 property used in the treatment of cylinders, 

 spheres and plates ; owing to the neglect of 

 this precaution, formulas based upon ap- 

 parent stresses, like those of Eankine, are 

 liable to give values often deviating twenty- 

 five per cent, from the truth. 



The formula for the design of columns, 

 long used in the United States under the 

 name of Gordon's formula or Eankine's 

 formula, has not been employed in Germany 

 to the extent that its value demands. The 

 author, however, emphasizes it as an im- 

 portant rule, and gives empiiical constants 

 for its use. He also states that the foi-mula 

 was first deduced by Navier ; on referring 

 to Navier's works this statement is not 

 found to be justified, it being only men- 

 tioned that the stress on the concave side 

 of the column is the sum of the stresses due 

 to direct compression and to lateral flexure, 

 while no formula similar to Gordon's is 

 established. 



On the whole, the perusal of the book 

 leaves the impression that the author has 



