240 SECTIONMECHANICS. 



manuscripts which startled me exceedingly. It contains the descrip- 

 tion of an experiment and its result, measuring the electrostatic 

 capacity of an insulated circular disc. That is one of the cases in 

 which the theory founded by Cavendish and Coulomb as developed 

 in the hands of the mathematicians who followed, allowed the result 

 to be calculated a priori, and I found the result agreed within, if I 

 remember rightly, one-half per cent, of Cavendish's measurements. 

 When I mention these cases of the measurement of electrical force 

 by Coulomb and Robinson, which has led to the true law of force and of 

 the measurement of electrostatic capacity, a subject which is the least 

 known generally, and held to be the most difficult, I have said enough 

 to show that we must not in this century claim all the credit of being 

 the founders of electrical measurement. 



The other main method of experimenting in connection with mea- 

 surement to which I have referred is illustrated also by Cavendish's 

 writings, that is the seeking for a zero. It is very curious, that while 

 Coulomb and Robinson by direct measurement of a continuously 

 varying quantity discovered the law of the inverse square of the dis- 

 tance, Cavendish, quite independently, pointed out by very subtle 

 mathematical reasoning that the law must either be the inverse square 

 of the distance, or must vary in a determinate manner from the law 

 of the inverse square of the distance if in a certain case, which he 

 defined, either a perfect zero of electric force is observed, or if instead 

 of a perfect zero any particular amount of electric force is observed. 

 It is quite clear from Cavendish's writings that he believed that perfect 

 zero would be found when the experiment should be made, but with a 

 caution characteristic of the man and also proper to his position as 

 an accurate philosopher and mathematician he never would state the 

 law absolutely. He had that scrupulous conscientiousness which pre- 

 vented him from guessing at the conclusion which no doubt he arrived 

 at. His mind was probably a great deal quicker than are many other 

 minds in which the conclusion is jumped at and given as if it were 

 proved, but he conscientiously avoided stating it as a conclusion, and 

 held it over until exact measurement should prove whether or not it 

 was to be concluded. 



The subject of measurement in this case of a null method pointed 

 out by Cavendish was this. If in the interior of a hollow electrified 



