216 [June 19, 



battery used on this occasion, and also to the fact of the magneto- 

 electric induction of the iron exterior. 



Experiment 9 shows that, on the introduction midway in the cir- 

 cuit of an escape (circuit derive), the resistance of which is equal 

 to half the circuit, the first zero is obtained at the rate of 2*78 revo- 

 lutions per second, or 5*56 reversals per second ; the introduction 

 of this escape about doubles the speed of transmission ; and thus, 

 by the establishment of a series of escapes judiciously along the cable, 

 the speed may be augmented to a very high degree without weaken- 

 ing the current too much for the purposes of telegraphy. Expe- 

 riments were then tried with currents of various durations ; and the 

 results of these experiments are very important, the highest speed 

 being obtained when the cable was connected to the battery for a very 

 short interval of time and immediately afterwards put to earth. In 

 this way, through the 540 miles, the speed of the wave was increased 

 from 1*326 to 3- 7. 



In the experiments in which resistances of various amounts were 

 added to one end of the cable, the consequent retardations agree very 

 nearly with the results obtained upon the Mediterranean cable. It 

 was found to be immaterial at which end of the circuit the resistance 

 was added : this, however, can only hold good with highly insulated 

 wires ; for it is evident, upon a little consideration of the matter, that, 

 where the line is imperfectly insulated, the resistance added at the 

 sending end will produce more retardation than if applied to the re- 

 ceiving end. 



In the experiments on the second zero and second maximum, it 

 is shown that, if the speed required to produce the first zero be taken 

 as unity, double that speed is necessary to produce the second maxi- 

 mum, and four times the speed to get the second zero. 



Notwithstanding the difficulties under which these experiments 

 were made (from the necessity of using a machine the rates of whose 

 motion could not be very accurately governed), the results are still 

 sufficiently accurate for all "practical" purposes of submarine tele- 

 graphy ; but such nice points as the retarding influence of the iron 

 covering cannot be inferred with any precision from these experi- 

 ments. It is certain, however, that in long cables the retarding 

 influence of the external iron covering is so small, compared with the 

 retardation due to electrical induction, that it may be neglected in 

 estimating the speed of the electric wave. 



