ii2 POPULAR LECTURES AND ADDRESSES. 



according to his provisional unit of time. His 

 measurement gives him a velocity in, let us say, 

 kilometres per this provisional unit of time, as 

 the value of the Siemens unit in absolute 

 measure. Then he knows from Lord Rayleigh 

 and Mrs. Sidgwick, that the Siemens unit in 

 absolute measure is 9,413 kilometres per mean 

 solar second ; and thus he finds the precise ratio 

 of his provisional unit of time to the mean solar 

 second. 



Still, even though this method might be chosen 

 as the readiest and most accurate, according to 

 present knowledge of the fundamental data, for 

 recovering the mean solar second, the method 

 by "v" is too interesting and too instructive, in 

 respect to elimination of the properties of matter 

 from our ultimate metrical foundations, to be 

 unconsidered. One very simple way of experi- 

 mentally determining "v" is derivable from an 

 important suggestion of Clark and Bright's paper 

 referred to above. Take a Leyden jar, or other 

 condenser of moderate capacity (for example, in 

 electrostatic measure, about 1,000 centimetres) 



