ELECTRICAL MEASUREMENT. 239 



inversely as the square of the distance, when the distance between 

 two bodies is varied. 



In physical science generally, measurement involves one or other of 

 two methods, a method of adjustment to a zero, or a what is called 

 a mill method, and again, a method of measuring some continuously 

 varying quantity. This second branch of measurement was illustrated 

 in Coulomb and Robinson's experiments, where the law according to 

 which the electric force varies, where the distance between the mutually 

 influencing bodies varies continuously, was determined. The other 

 mode of experimenting in connection with measurement, is illustrated 

 by another exceedingly important subject, bearing upon electrical 

 theory, and that is the evanescence of electrical force in the interior of 

 a conductor. Both kinds of measurements were practiced by Cavendish 

 in a very remarkable manner, and I look forward with great expecta- 

 tion to the results we are soon to have of Cavendish's work. One 

 most interesting result which will follow from the Cavendish labo- 

 ratory in Cambridge, from its director Professor Clerk-Maxwell, and 

 from the relationship thus established between the physical laboratory 

 of the University of Cambridge, and its director on the one hand, and 

 the munificent founder of the institution, the Duke of Devonshire, on 

 the other hand. The Cavendish manuscripts still remain in that 

 family, and are, I believe, at present in the possession of the Duke of 

 Devonshire, and have been by him put into the hands of Professor 

 Clerk-Maxwell for the purpose of having either the whole, or extracts 

 from, published, which may be found to be of scientific interest at the 

 present day. The whole of them, no doubt, had great scientific 

 interest at one time. A large part of these manuscripts, I believe, will 

 be found to be excessively interesting even now, and from something I 

 heard a few days ago from Professor Maxwell, when he was here on 

 the opening day of this exhibition, I learnt that much more than was 

 even imagined is to be found in these manuscripts, and particularly the 

 whole branch of electrical measurements worked out, from the mea- 

 surement of electrostatic capacity. The very idea of measuring electro- 

 static capacity in a definite scientific way is, as it now turns out, due to 

 Cavendish. A great many years ago, in 1846 or 1847, when the 

 Cavendish manuscripts were in the hands of Sir Wm. Snow Harris, 

 at Plymouth, I myself found one paper, out of a box full of unsorted 



