REVIEWS 



Collected Scientific Papers. By J. H. Poynting, Sc.D., F.R.S. [Pp. 

 xxxii4- 768.] (Cambridge: at the University Press, 1920. Price 

 375. 6d. net.) 



The collected papers of the late John Henry Poynting, Mason Professor of 

 Physics in the University of Birmingham, have been published in book form 

 with a view to perpetuate his memory, a committee formed in 1914 to 

 consider how best to establish a suitable memorial having decided that this 

 was the best form which it could take. A fund was opened for this purpose, 

 and the volume under review, produced with the customary excellence of 

 the Cambridge University Press, is the result. 



The work has been edited by two of Prof. Poynting's colleagues, G. A. 

 Shakespear and Guy Barlow, who have corrected a few arithmetical and 

 other mistakes. All the important corrections are indicated in footnotes. 

 The papers have been arranged in groups, so as to bring together all the 

 papers on kindred subjects ; within each group the papers have been 

 arranged in chronological order. The popular presentations of subjects 

 previously treated strictly scientifically have, however, been collected to- 

 gether with various addresses and other articles of a popular nature, in the 

 last section of the volume. The papers have been preceded by biographical 

 and critical notices by Sir Oliver Lodge, Sir Joseph Larmor, Sir J.J. Thom- 

 son and G. A. Shakespear. 



In Part I are collected the papers dealing with the balance and gravita- 

 tion, of which the most important is the Phil. Trans, paper of 1892, " On a 

 Determination of the Mean Density of the Earth and the Gravitation Con- 

 stant by means of a Common Balance." Poynting's method of determining 

 the mean density was to measure the attraction between two known masses 

 by finding the increase in the weight of one of them when the other was 

 brought underneath it. Merely regarded as a development of the technique 

 of accurate weighing, this work was of great importance. The manner in 

 which disturbing causes were successively tracked down and eliminated 

 should prove very instructive to students of physics. Although the experi- 

 ments were commenced in 1878, it was not until the end of twelve years' 

 work that Poynting obtained a result with which he was satisfied. By that 

 time, however, he tacitly admitted that the compact torsion-balance 

 apparatus designed by C. V. Boys was more suitable for the purpose. His 

 final determination of the mean density, 5 '49, is nevertheless entitled to a 

 relatively high weight in any discussion of the most probable value to be 

 deduced from the various determinations which have been made. 



Part II contains the papers on electricity, and of these the two Phil. 

 Trans, papers of 1884 and 1885, " On the Transfer of Energy in the 

 Electromagnetic Field " and " On the Connection between Electric Current 

 and the Electric and Magnetic Inductions in the Surrounding Field " are the 

 most important. They form, in fact, Poynting's most valuable contribution 

 to physical science. "Their importance is very succinctly summarised by Sir 

 Joseph Larmor : " Nobody before Poynting seems to have thought of 

 tracing the flux of energy in a medium elastically transmitting it, and where 

 the whole process is therefore exposed to view. The line of flow is a ray in 



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