THE EXACT MEASUREMENT OF PHENOMENA, 355 



pendulum, and the principle of repetition already described 

 (PP- 339> 353)- As regards short intervals of time, it has 

 already been stated that Sir George Airy was able to 

 estimate a difference of 2^ seconds per day, between two 

 pendulums with an uncertainty of less than '01 of a second, 

 or one part in 8,640,000, an exactness, as he truly remarks, 

 'almost beyond conception 11 '. The ratio between the mean 

 solar and the sidereal day, too, is known to about one part 

 in one hundred millions, or to the eighth place of decimals 



(P- 337)- 



Determinations of weight seem to come next in exact- 

 ness, owing to the fact that repetition without error is 

 applicable to them (p. 340). An ordinary good balance 

 should show about one part in 500,000 of the load x . The 



finest balance employed by M. Stas, turned with of a 



OO 



milligramne, when loaded with 25 grammes in each pan, 

 that is, with one part in 825,000 of the loady. But balances 

 have certainly been constructed to show one part in a 

 million 2 , and Eamsden is commonly said to have con- 

 structed a balance for the Eoyal Society, to indicate one 

 part in seven millions, though this is hardly credible. 

 Professor Clerk Maxwell takes it for granted that one 

 part in five millions can be detected, but we ought to 

 discriminate between what a balance can do when first 

 constructed, and when in continuous use. 



Determinations of lengths, unless performed with extra- 

 ordinary care, are open to much error in the junction of 

 the measuring bars. Even in measuring the base line of 

 a trigonometrical survey, the accuracy generally attained 

 is only that of about one part in 60,000, or an inch in the 



u 'Philosophical Transactions,' (1856), vol. cxlvi pp. 330-1. 

 x Thomson and Tait, 'Natural Philosophy,' vol. i. p. 333. 

 y 'First Annual Report of the Mint,' p. 106. 

 z Jevons, in Watts' ' Dictionary of Chemistry,' vol. i. p. 483. 



A a 2 



