78 POPULAR LECTURES AND ADDRESSES. 



could be found to defend it. As for our electro- 

 static apparatus, I confess that I do not know the 

 capacity of a single one of the two or three dozen 

 Leyden jars, which in 1846 I inherited, in the 

 Natural Philosophy apparatus-room of the 

 University of Glasgow, or which I have made 

 from time to time during the thirty-seven years 

 passed since that date. I would fain hope that I 

 am singular in such a confession, and that no 

 other professor of Natural Philosophy in the 

 world would let a Leyden jar be put on his 

 lecture-room table without being able to tell 

 his students its capacity in absolute measure. 

 The reckoning of Leyden-jar capacity in square 

 inches of coated glass thickness and specific 

 inductive capacity not stated ought to be as 

 much a thing of the past as is the reckoning of 

 resistances in terms of a mile of wire weighing 

 fourteen grains to the foot, of ordinary 

 commercial copper, specific resistance not stated 

 perhaps 45 per cent. ? or 70 per cent. ? or 98 

 per cent. ? of the conductivity of pure copper. 

 And as to practical measurement of electromotive 



