414 Notices respecting New Boohs. 



is something amusing in the contrast between the dimensions of 

 the apparatus used by Professors Poynting and Boys. Thus, the 

 former uses two attracted spheres made of an alloy of lead and 

 antimony, the masses (not quite equal) being each over 21 1 kilo- 

 grammes with diameters each nearly 16 centimetres, while one 

 large attracting sphere has a mass of over 153 kilogrammes with 

 a diameter of 30 1 centimetres, the other being of half the size ; 

 while of Professor Boys's apparatus it is sufficient to say that it is 

 of the "waistcoat pocket" order of magnitude. 



The author very justly summarizes his account of the previous 

 methods for the determination of the mean density (those depend- 

 ing on plumb-line experiments, involving the calculation of the 

 attraction of mountain masses, &c), when he says (p. 39), " All this 

 tends to confirm the conclusion that our knowledge of the dis- 

 tribution of the terrestrial matter is not yet sufficiently exact to 

 enable us to obtain good values of the mean density of the Earth 

 from the observed attraction of terrestrial masses. Rather must 

 we assume the mean density from laboratory experiments, . . . ; " 

 and of this latter kind are his own and those of Professor Boys. 

 The former, or " Common Balance Method," may be roughly 

 described as consisting in the suspension of two nearly equal 

 heavy spheres from the ends of the arms of a common balance, 

 while a very large attracting sphere is brought close under each of 

 the former spheres successively, its attraction (chiefly on the sphere 

 just above it) altering the position of equilibrium of the balance. 



Incidentally (p. 78) Professor Poynting does a service to experi- 

 mentalists in advocating the great value of what he calls " the 

 double-suspension mirror method " of measuring small deflexions, 

 as compared with the method of attaching the mirror to the 

 moving needle commonly employed in electrometers and galvano- 

 meters, the latter multiplying the angular deflexion by 2 only, 

 while the former, as employed by Professor Poynting, multiplied 

 the angular deflexion of the balance by 150. 



With a view to a second edition of this Essay, it may be well to 

 point out a few things that might be altered with advantage. 

 Doubtless, from the author's long familiarity with his symbols, he 

 has not thought it worth while to notice that the symbols M and A 

 in the mathematical investigation (pp. 109 &c.) are used each in 

 two senses even in the same equation : at first they are, re- 

 spectively, the mass of the large attracting sphere and the mass 

 of one of the attracted spheres with a narrow diametral cylinder 

 removed ; while immediately afterwards (and in the same equation, 



M 

 end of p. 112) A stands for ^ , where now M means the "mass 



deflexion," and E the " rider deflexion." This is, for a time, 

 somewhat confusing. Finally, a few misprints may be noted : 

 line 16, p. 109, for n read n' ; line 14, p. 110, for fig. 1 read 



J T 



fig. 17; line 1, p. Ill, for 1— „- read I— -~; line 3, p. 115 



brackets omitted in denominator at left side ; together with a few 

 others too obvious to escape notice. 



