ELECTRICAL UNITS OF MEASUREMENT. 119 



the coil and condenser are of dimensions con- 

 venient for the actual experiment, as we terrestrials 

 might do it. The velocity of the end of that hand 

 is "v." There he has this wonderful quantity 

 " v" He has a hand going round in a certain 

 time, and he knows that if that hand be of the 

 calculated length, the velocity of the end of it 

 is " v" This is interesting and instructive, and 

 though I do not for certain know that it is 

 very practicable, it is still, I believe, sufficiently 

 so to be worth thinking of. I think it will be 

 one of the ways of determining this marvellous 

 quantity " v" 



It is to be hoped that before long " v" will be 

 known, in centimetres per mean solar second, 

 within i/io per cent. At present it is only known 

 that it does not probably differ 3 per cent, from 

 2*9 x io 10 centimetres per mean solar second. 

 When it is known with satisfactory accuracy, an 

 experimenter provided with a centimetre measure 

 may, anywhere in the universe, rate his experi- 

 mental chronometer to mean solar seconds, by 

 the mere electrostatic and electro-magnetic opera- 



