176 Reviews — Prof. Poynting — Mean Density of the Earth. 



the mean density from laboratory experiments, and use the obser- 

 vations of terrestrial attractions for the converse problem of deter- 

 mining the distribution of terrestrial mass." The whole part abounds 

 with passages such as this, which, condensed as sentences from Bacon's 

 Essays, might form chapter headings for many a work to come. It 

 closes with the following "summary of results hitherto obtained : " — 



With Part II. begins the more strictly original half of the 

 essay, " A determination of the mean density of the earth and 

 the gravitation constant by means of the common balance." 



The principle of the method is as follows : Two equal masses 

 A and B, are suspended one from each end of the beam of a 

 very delicate balance. A large sphere M is brought first under 

 the mass A, and then under the mass B. Its attraction being 

 removed from A and added to B, it follows that the beam turns 

 through nearly double the angle due simply to the attraction 

 of M on one mass alone. Of course when M is under A it will at 

 the same time attract B, and when under B it will attract A, and 

 these cross-attractions must be taken into account. So also would 

 the attraction of M on the balance-beam and suspending- wires have 

 to be allowed for ; but this operation (most difficult on account of 

 their irregular shape) is avoided by repeating the experiment after 

 raising the masses A and B through known equal distances, without 

 altering the length or form of the suspending wires. It is evident 

 that the attraction of M on the beam and wires is the same in both 

 experiments, and the change in the attraction is therefore due only 

 to the change in the distances of ^ and B from M. From this 

 change in attraction, the attraction at any distance can be found. 



The balance, whose beam is a little over 4 feet long, was erected 



