2 



Mr. J. H. Poynting on a method of [Nov. 21, 



The following Papers were read : — 



I. " On a method of using the Balance with great delicacy, 

 and on its employment to determine the Mean Density of 

 the Earth." By J. H. Poynting, B.A., Fellow of Trinity 

 College, Cambridge, Demonstrator in the Physical Labora- 

 tory, Owens College, Manchester. Communicated by Pro- 

 fessor B. Stewart, F.R.S. Received June 21, 1878. 



[Plate I.] 



In the ease and certainty with which we can determine by the 

 balance a relatively small difference between two large quantities, it 

 probably excels all other scientific instruments. 



By the use of agate knife edges and planes, even ordinary chemical 

 balances have been brought to such perfection that they will indicate 

 one-millionth part of the weight in either pan, while the best bullion 

 balances are still more accurate. The greatest degree of accuracy 

 which has yet been attained was probably in Professor Miller's weigh- 

 ings for the construction of the standard pound, and its comparison 

 with the kilogramme, in which he found that the probable error of a 

 single comparison of two kilogrammes, by Gauss's method, was 

 1400 1 0000 th part of a kilogramme.* (" Phil. Trans.," 1856.) 



But, though the balance is peculiarly well fitted to detect the 

 relatively small differences between large quantities, it has not hitherto 

 been considered so well able to measure absolutely small quantities 

 as the torsion balance. The latter, for instance, was used in the 

 Cavendish experiment; when the force measured by Cavendish was 

 the attraction of a large lead sphere upon a smaller sphere, weighing 

 about 1-| lbs., the force only amounting to B o 6o o o o ^ a P ar ^ °^ ^ n ^ s 

 weight, or about 50 1 0O th part of a grain. 



The two great sources of error, which render the balance inferior to 

 the torsion balance in the measurement of small forces, are : — 



1. Greater disturbing effects produced by change of temperature, 

 such as convection currents and an unequal expansion of the two arms 

 of the balance. 



2. The errors arising from the raising of the beam on the support- 

 ing frame between each weighing, consisting of varying flexure of the 

 beam and inconstancy of the points of contact of the knife edges and 

 planes. 



The disturbances due to convection currents interfere with the 

 torsion balance as well as with the ordinary balance, though they are 



* Even so far back as 1787, Count B-umford used a balance which would indicate 

 one in a million and measure one in seven hundred thousand. (" Phil. Trans.," 



1709.) 



