376 THE ELECTRIC TELEGRAPH. 



perature giving any other resistance is easily calculated by 

 means of the coefficients given in the table at p. 267. 



A more handy arrangement still is by substituting a 

 resistance, R, equal in every respect to R', in fact, by a 

 second resistance-thermometer. This is immersed in a vessel 

 of water whose temperature is changed until the electric 

 equilibrium is established ; the temperature of the water 

 gives then, without any reduction whatever, by means of a 

 mercury thermometer, the temperature of the distant coil R'. 

 Only that portion of the apparatus on the left-hand side 

 of the dotted line a b is in the testing-room, that on the 

 right hand is outside. Such an arrangement might be used 

 with good results for ascertaining the temperature of the 

 sea at different depths. In this case all on the right-hand 

 side ofab would be submerged, that on the left on board. 

 With a number of these instruments, placed between the 

 layers of cable on board the ship Queen Victoria it was 

 observed, after the cable had been stowed a few days, that 

 the upper part increased in temperature at the rate of about 

 3 F. daily, until 86 F. was reached, while the lower part 

 retained the temperature of the hold and water. 



The obstinate doubts which were urged at the time against 

 the truth of these results were silenced signally when water 

 of 42 F. was pumped upon the cable, and flowed out at the 

 bottom at a temperature of 72 F. This shows the necessity 

 of keeping an iron-covered cable, which has once been under 

 water, always under water, even during tjie transport, although 

 it may be attended with some inconvenience. 



The tests which are made at the sheathing works are of 

 the same kind as those made at the gutta-percha works. 

 Faults sometimes occur ; but these are easy to determine, as 

 the operator is in possession of both ends of the cable. 



In order to facilitate th,e discovery and location of faults 

 in sheathed cables, Mr. "Willoughby Smith has invented and 

 patented the idea of serving the core with tanned hemp 

 instead of tarred hemp, which he professes to have found has 

 an inclination to temporarily mend small faults which might 

 develop themselves when submerged. 



