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XXXVIII. Notices respecting New Boohs. 



The Mean Density of the Earth : an Essay to which the Adams 

 Prize tuas adjudged in 1893 in the University of Cambridge. By 

 J. H. Poynting, Sc.D., F.B.S. (Charles Griffin and Company.) 



rPHE study of this work of Professor Poynting will be of the 

 -^ utmost value not only to anyone who proposes to engage on 

 the special subject with which it deals, viz., the combined mathe- 

 matical and experimental determination of the constant of gravita- 

 tion — bat also to the fairly large class of investigators who, in other 

 experimental researches, have to deal with the measurement of ex- 

 tremely small quantities and with the action of disturbing influences 

 both small and great. The chief value, indeed, of the Essay consists 

 in the help which it affords in anticipating and allowing for such 

 disturbances, and in the fairly complete historical resume of the work 

 which has been done, with the same object, by previous investigators. 



A work of this kind would lose much of its value if it did not 

 contain a fair record of failures as well as of successes ; and the 

 author has not omitted to point out the defects which experiment 

 from time to time revealed in his originally projected methods — 

 as, for example, when in 1888, after he had supposed that the 

 work was finished, he was led to suspect the existence of a tilting 

 of the whole floor of the room in which his balance was placed by 

 the moving of the large attracting mass from one position to 

 another; an unfortunate circumstance which necessitated the 

 employment of a second attracting mass whose action was such as 

 to diminish the effects which were to be observed. 



The author is to be congratulated on the strictly scientific title 

 under which he describes his work — " The Determination of the 

 Mean Density of the Earth," or " The Determination of the Con- 

 stant of Gravitation," instead of the utterly unmeaning " Deter- 

 mination of the Weight of the Earth," which is found in even such 

 a work as Arago's ' Popular Astronomy,' and which is a characteristic 

 of too much of our modern popular science a la mode. Have we 

 not seen in some old and popular treatise a picture of " the room 

 in which Mr. Baily weighed the Earth " ? It is to be hoped that 

 some day our leading authorities will be induced to abandon that 

 fatal dogma which is still, unfortunately, " of great emolument " — 

 that Science, to be popular, must, above all things, be inaccurate. 



"We have said above that the chief value of the work consists in 

 its helpfulness and suggestiveness, although it is tolerably certain 

 that, with all the precautions which the author has taken to ensure 

 success, the value which he has found for the mean density (viz. 

 5*493) must be very near the truth. He himself points out (p. 107) 

 that with all apparatus the greater the size the greater are the 

 errors produced by air-currents, a fact of which he was unaware 

 when his apparatus was designed ; and that, if he were to start 

 with a new design, he would certainly make the whole arrange- 

 ment on a smaller scale — a great change which was advocated and 

 justified by Professor Boys in a preliminary paper on the Cavendish 

 experiment, read before the Royal Society in May 1889. There 



