356 Mr. C. Vernon Boys [June 8, 



Owing to the universal character of the constant G, it seems to 

 me to be descending from the sublime to the ridiculous to describe 

 the object of this experiment as finding the mass of the earth, or the 

 mean density of the earth, or, less accurately, the weight of the earth. 

 I could not lecture here under the title that has always been chosen 

 in connection with this investigation. In spite of the courteously 

 expressed desire of your distinguished and energetic secretary, that 

 I should indicate in the title that, to put it vulgarly, I had been 

 weighing the earth, I could not introduce as the object of my work 

 anything so casual as an accidental property of an insignificant planet. 

 To the physicist this would be equivalent to leaving some great 

 international conference to attend to the affairs of a parish council. 

 That is the business of the geologist. The object of this investiga- 

 tion is to find the value of G. The earth has no more to do with it 

 than the table has upon which the apparatus is supported. It 

 does interfere, and occasionally, by its attraction, breaks even the 

 quartz fibres that I have used. The investigation could be carried on 

 far more precisely and accurately on the moon, or on a minor planet, 

 such as Juno ; but as yet no means are available for getting there. 



I shall not have time to-night to describe the work of former 

 investigators, and for this there is little need, since it is all collected 

 in Poynting's Adams prize essay, "On the Mean Density of the 

 Earth," published this year. I cannot even find time to explain in 

 more than the merest outline what I have done to develop the 

 apparatus of Cavendish, so that he would hardly recognise in my 

 glorified bottle-jack the balls and lever which have made his name 

 famous. The following table, given by Poynting, however, represents 

 the results of the labours of investigators up to the present time. 



In connection with this table I cannot lose the opportunity of 

 quoting Newton's extraordinary prophecy, marvellous in that without 

 any direct knowledge he gave a figure which was nearer the truth 

 than that found by many of the experimenters that came after him. 

 The passage is as follows : — 



" Unde cum Terra communis suprema quasi duplo gravior sit 

 quam aqua, et paulo inferius in fodinis quasi triplo vel quadruplo 

 aut etiam quintuplo gravior reperiatur; verisimile est quod copia 

 materia totius in Terra quasi quintuplo vel sextuplo major sit quam 

 si tota ex aqua constaret ; praBsertirn cum terrain quasi quintuplo 

 densiorem esse quam Jovem jam ante ostensum sit." * 



I have placed on the wall the diagram of the apparatus which I 

 showed in action when lecturing here upon quartz fibres five years 

 ago. With this I was able, for the first time, to show to an audience 

 the effect of the very small attraction exerted between a 2-inch 

 cylinder of lead and a little one weighing only a gramme or 15 

 grains. The apparatus which I have to describe to-night is the same 

 in principle, the main distinction being that it is so designed and 



* Newton's ' Principia,' second edition, 1714, p. 373, line 10. 



