174 Mr. F. Jenkin on the Question of the 



may, however, observe that I did not think it necessary to give 

 the exact dates at which any of the mile or kilometre units 

 were introduced after those which I was informed were used in 

 1847 by the Electric and International Company. 



Dr. Siemens further criticises my statement that, until about 

 the year ] 850, measurements of resistance were chiefly confined to 

 the laboratory; but I confess I think his statement that in 1848 

 they began to make resistance-coils rather proves than disproves 

 my assertion, especially as I had mentioned that the Electric 

 and International Company had coils in use in 1847. 



Next he complains that I do not mention the complete set of 

 resistance-coils made in 1859. But as resistance-coils were then 

 common, I do not see why they should have been mentioned. 

 Does Dr. Siemens mean that he then weighed the coils, instead 

 of using Wheatstone's balance or other electrical adjustment ? 

 1 had in 1859 one set of coils adjusted by the Wheatstone's 

 balance to within Ol per cent., which I use to this day, made 

 by White, under Professor Thomson's directions; and many 

 other sets were in use at the time. These were the coils I used 

 as standards at Birkenhead (not Dr. Siemens' s coils, of which 

 I had only very rough specimens of the so-called German mile). 



I did not mention this set of practical coils, or any others, my 

 object being to mention new units. I only speak of them now 

 to show that my silence cannot possibly have proceeded from 

 any hostile animus. 



Dr. Siemens says my historical sketch is very incomplete; 

 but, beyond the correction in the description of one comparatively 

 unimportant unit, he does not add one definition to those I 

 enumerated, although I quite admit that in all probability some 

 omissions must have occurred. As to Marie Davy's prior pro- 

 posal for a mercury standard, my own statement is that, though 

 not first in order of time, Dr. Siemens merits especial recogni- 

 tion, owing to the manner in which his proposal was carried 

 out. Is not this what Dr. Siemens claims ? The Report was 

 not specially on his unit; and he may observe that Ohm, Wheat- 

 stone, Weber, and Thomson, and others of equal rank, have ne- 

 cessarily each but a few lines in the short sketch I wrote. 



Next Dr. Siemens complains that I mentioned neither the 

 fact that in 1859 the relative resistances of the conductor and 

 insulator were given in mercury units, nor the method fol- 

 lowed to measure the resistance which the insulating covering 

 offered to the electric current and to compare it with that cal- 

 culated from the specific resistance of the insulated material. 



This subject was unconnected with the introduction of any new 

 unit, and was therefore omitted as irrelevant. 



If I had mentioned the subject at all, I should first have 



