August 2, 1694] 



NA TURE 



331 



parish council. That is the business of the geologist. The 

 object of these investigations is to find the value of G. The 

 I earth has no more to do with the investigation than the table 

 I has upon which the apparatus is supported. It does interfere 

 t and occasionally, by its attraction, breaks even the quartz fibres 

 I that I have u<ed. The investigation could be carried on far 

 .Tiore precisely and accurately on the moon, or on a minor 

 ' planet, such as Juno ; but as yet no means are available for get- 

 I ting there. 



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

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

 lected inPoynting's Adams prize essay " On the Mean Density 

 I of the Earth," published this year. I cannot even find time to 

 i explain in more than the merest outline what I have done to 

 I develop the apparatus of Cavendish, so that he would hardly 

 I recognise in my glorified bottle-jack the balls and lever which 

 j have made his name famous. The following table, given by 

 I Poynting, however, represents the results of the labours of in- 

 vestigators up to the present time. 



Summary of Results hitherto obtained. 



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 e.xperimenters that 

 came after him. The passage is as follows : — 



" Unde cum Terra communis suprema quasi duplo graviorsit 

 quam aqua, et paulo inferius in fodinis quasi triplo vel 

 quadruple aut etiam quintuple gravior reperiatur ; verisimile 

 est quod copia materia totius in Terra quasi quintuplo vel sex- 

 tuple major sit quam si tota ex aqua constaret ; prjesertim cum 

 terram quasi quintuplo densiorem esse quam Jovem jam ante 

 ostensum sit." (Newton's "Principia," 2nd edition, 1714, 

 p. 373, line 10.) 



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

 I showed in .action when lecturing here upon quartz fibres five 

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

 I an audience the effect of the very small attraction exerted be- 

 I tween a two-inch cylinder ofleadanda little one weighing only 

 1 a gramme or fifteen 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 constructed that I can tell pre- 

 cisely where every gravitating particle is placed. In the design 

 of this apparatus I have been, .as everyone will admit, bold — 

 most would have preferred the word reckless ; but knowing the 

 truth of the principles which I had developed, and having faith 

 and confidence in the quartz fibre, I deliberately chose to reduce 

 all the dimensions to an extent which caused the forces, and 

 especially the couples, to be insignificant in comparison with 

 any which had been within the reach of the experimenter 



hitherto. The whole difficulty of Cavendish, Reich, and 

 Baily had been to measure so minute an effect ; instead of 

 increasing this, I diminished it enormously, being satisfied that I 

 should be able to make a proportionately more accurate measure 

 by so doing. Cornu reduced the dimensions to one-quarter; I 

 have reduced the chief one to one- eightieth. Cavendish had a force 

 equal to 1/3650 grain's weight to measure ; I have less than a 

 five-millionth. liy the use of the long lever, Cavendish had the 

 effect of a force of l/ioo grain's weight on an arm an inch long ; 

 I have less than a twelve-millionth of a grain on an arm of 

 that length. His forces were fourteen hundred times as great 

 as mine ; his couples or twisting forces were a hundred and 

 twenty thousand times as great. One advantage gained by the 

 use of small apparatus, in which alone the attracting balls can 

 be made large compared with the length of the beam, is the 

 increased sensibility, the greater angle of deflection produced 

 by the attractions when the period of oscillation is the same. 

 This is more especially the case in my apparatus where the two 

 sides are at different levels. But the greatest advantage is in a 

 direction whence it might least be expected. In spite of every 





By permission of the Engineer.'l ^^-^ y 



Fig. .. 



endeavour that may be made to keep the air quiet, to exclude 

 draughts, to keep all the apparatus at one temperature in a 

 vault of constant temperature, infinitesimal differences must 

 exist ; one side of the apparatus must be hotter than the other, 

 though no thermometer could be made which would detect the 

 difference. In consequence of this difference of temperature 

 the air circulates, and so creates a draught which blows upon 

 the mirror and the suspended balls. Now I have shown that in 

 apparatus geometrically similar these disturbances are likely lo 

 be in the proportion of the seventh power of the linear dimen- 

 sions, while the gravitational couples vary only as the fifth 

 power ; the relative disturbances are therefore likely to be in 

 the proportions of the squares of the linear dimensions, so that 

 if we make our apparatus ten times as large, the mirror is likely 

 to be one hundred times as unsteady. In addition to this, the 

 time needed to bring the apparatus to a steady state is far 

 greater with large apparatus. After making the geometrical 

 measures I leave my apparatus, small as it is, three days, if 

 possible, before observing deflections and periods. 

 The diagram (Fig. i) is a vertical section through the appa- 



NO. 1292, VOL. 50] 



