246 SECTION MECHANICS. 



the capacity of the Leyden jars in that way, and in future when anyone 

 goes to buy a Leyden jar of an optician, let him tell you to give him 

 one of one or two metres or whatever it may be, and require him to 

 find out how to produce it. I give that as a hint to anyone interested 

 in electrostatic apparatus, or in the furnishing of laboratories. There 

 is no likelihood that he will understand what you mean, but perhaps if 

 you teach him a little he will soon come to understand it, and I hope in ten 

 years hence, in every optician's shop where Leyden jars are sold, there 

 will be a label put on each jar saying the capacity is so many centi- 

 metres. It could be done to-morrow. We have all the means of 

 doing it, only we have not the knowledge. 



The relation between electrostatic measurement and electro-magnetic 

 measurement is very interesting, and here from the supposed unin- 

 teresting realms of minute and accurate measurement we are led to the 

 depths of science, and to look at the great things of Nature. Thrse 

 old measurements of Weber led to an approximate determination of 

 the particular velocity at which the electro -magnetic resistance is 

 numerically equal to the electrostatic conducting power of a wire. The 

 particular degree of resistance of a wire which shall be such that the 

 velocity which measures the resistance in electro-magnetic measure 

 shall be the same as the velocity which measures the conducting power 

 in electrostatic measure, was worked out by Weber, and he found that 

 velocity to be just about 300 kilometres per second. I unhappily have 

 British statute miles in my mind, through the misfortune of being born 

 thirty years too soon, and I remember the velocity of light in British 

 statute miles. That used to be considered about 192,000 miles per 

 second, but more recent observations have brought it down to about 

 187,000. Now I think the equivalent of that in metres is about 300 

 kilometres per second, and that was the number found by Weber. 

 Professor Maxwell gave a theory leading towards a dynamical theory 

 of magnetism, part of which suggested to him that the velocity for 

 which the one measure is equal to the other in the manner I have 

 explained should be the velocity of light. This brilliant suggestion has 

 attracted great attention, and has rendered it an object of intense 

 interest, not merely for the sake of accurate electro-magnetic and 

 electrostatic measuring, the measuring with great accuracy the relation 

 between electrostatic and electro-magnetic units, but also in connection 



