654 Notices respecting New Books. 



How many distinct types of wave-motion are 

 possible in free cether f 

 28. If, as is here suggested, neither the magnetic nor the 

 electric vector, nor yet the vector product of these two, 

 corresponds to a bodily flow of aether, it would seem a likely 

 conclusion that no purely electromagnetic phenomenon in- 

 volves the bodily displacement of any aether-element ; that 

 the aether remains, in the grosser sense, permanently at rest, 

 experiencing only changes of " polarization." (The rapidly 

 travelling compressicnal waves, which I have ventured to 

 suggest as affording a possible basis for gravitation, involve 

 only such slight deformations of the aether that they need not 

 be supposed to interfere with electromagnetism.) But if 

 aetherial waves of bodily transverse displacement have no 

 part in electromagnetic or in gravitational phenomena, it is 

 natural to inquire whether the aether is (as a frictionless 

 fluid would be) incapable of transmitting waves of that type 

 — or if such waves, having been generated by a suitable 

 agency, would be propagated through the aether with a 

 definite velocity, are we to infer that they are non-existent 

 because no system of moving electrons is competent to pro- 

 duce them ? If I may venture an opinion on so recondite a 

 matter, it seems to me not improbable (in view especially of 

 the considerations put forward in this paper) that by no 

 manipulation of ordinary matter could aetherial waves of 

 bodily transverse displacement be initiated ; but in that case 

 it might well be that waves of the type referred to, if existing 

 in the aether, would fail to produce, through their interaction 

 with atomic matter, any sort of observable phenomenon. At 

 this point it would seem prudent to leave the question as to 

 whether waves of transverse displacement are being propagated 

 through the aether or not. 



My' thanks are due to Prof. G. M. Minchin, F.R.S., who 

 most kindly read the MS. of this paper. His valuable 

 criticism having led me to rewrite certain sections, it may 

 be hoped that some obscurities have thus been removed. 



LVIII. Notices respecting New Books. 

 General Physics. Henry Crew, Ph.D. New York: The Mac- 

 Millan Co., 1908. Price 12s. net. 



THIS is intended as a first year university course in physics. It 

 obviously consists of the amplified outline of a course of lectures ; 

 and therefore we miss the development of the measuremental (as 

 distinct from the experimental) side of the subject which we like 

 to see in a book intended for students' use. The first half of the 

 book consists of mechanics. We are not sure that the experi- 

 mental illustrations chosen are always the best. The case in which 



