Notices respecting Neiv Books. 569 



tropic in the sense assumed, say, by Thomson and Tait in 

 developing the equations you use ; they are granular. 

 Probably the only way to find s for rock is to measure it 

 directly. It is difficult to make a direct measurement of k, 

 and hence observed velocities of wave-transmission may prove 

 the best data. 



You speak of a possible error in the results due to viscous 

 yielding introducing a time element. This is referred to in 

 the records, the statement being made that no sensible viscous 

 yielding w;is observed in any of the specimens except marble, 

 and in this it was small. 



I have been much interested in the subject of elastic con- 

 stants and the application of these equations for many years, 

 and hope to be able to write something on it at an early date. 



Should you think it worth while, I should be glad to have 

 you send the above to the Phil. Mag. Yours very truly, 



Thomas Gray. 



LX. Notices respecting New Books. 

 The Elements of Physics. By Henry Crew, Ph.D., Prof, of Physics 

 in Northwestern University, New York. The Maemillan Co., 1899. 

 r PHIS treatise is written for the use of students in high schools, 

 -"- and aims at presenting the elementary parts of the different 

 branches of physics in a strictly consecutive order. In his some- 

 what remarkable preface the author contends that physics is not 

 a series of disconnected subjects, and that " when a student passes 

 from mechanics to sound, or from heat to electricity, he must not 

 feel that he is taking up a new subject." If this meaus that the 

 student is to realize that the new branch still treats of matter and 

 energy, the author has accomplished his aim ; but if the intention 

 is to arrange in all cases a passage as consecutive as that from 

 elasticity and wave-motion in mechanics to the phenomena of 

 sound, we must confess that the attempt appears to us an un- 

 successful one. The subject of heat follows after sound, and is 

 commenced by a definition of temperature and a description of 

 thermometry; the fact that bodies change in size by heat is just 

 mentioned in order to explain the action of a thermometer, but the 

 study of expansion comes much later, after the student has been 

 casually informed, in order to explain convection, that the density 

 of hot air is less than that of cold air. Again, the student passes 

 directly from the description of the steam-engine to the funda- 

 mental phenomena of magnetism, from Ohm's law to the rectilinear 

 propagation of light. In the latter case there is certainly an 

 introductory paragraph indicating that both light and sound are 

 phenomena perceived by special sense- organs. 



There is one stage in the study of mechanics where the corre- 

 lation of the physical sciences desired by the author may be 

 appropriately and easily introduced. It occurs just after the 

 treatment of the laws of motion and the derivation of the mathe- 

 matical expression for the kinetic energy of a body, a dose of 



