SCIENCE AND PRACTICE. 403 



directly to earth. ; for which purpose a contact, c, will have 

 to be provided. The transmission from ship to shore will be 

 done by simply changing the electro-motive force of the 

 battery either by increasing or by lessening it by which 

 the current in the instrument G will be correspondingly 

 altered, and the signals be indicated by the movements of 

 the needle. Each time this occurs, of course, a new charge 

 and discharge will take place which will prevent the rate of 

 speaking ever exceeding that which, with the same receiving 

 apparatus, wculd be attainable were the resistance R not in 

 circuit. 



The shore telegraphs to the ship, by pressing down the 

 key K, substituting thereby the resistance R' for R, altering 

 therefore the total resistance in the battery circuit. These 

 alterations are responded to by the needle of G'. 



A great benefit of this system is that, should a fault occur 

 in the cable even while a correspondence is being carried on, 

 it will become at once evident to the ship ; and, if a con- 

 siderable fault, to both ship and shore. Unfortunately, how- 

 ever, beyond the mere qualitative knowledge that the cable 

 is good and as a means of correspondence, the deflections of 

 the galvanometer on shore are totally useless. As data for 

 calculating the distance of a fault they are valueless, because 

 faults can only be determined by help of the resistance of 

 the copper conductor ; and this is so overpowered by the 

 resistance R, which is in the same circuit, that were the fault 

 to occur anywhere at A or at B, or midway between the two, 

 the result, so far as the galvanometer G is concerned, would 

 be absolutely the same. 



The method is unquestionably one of the most ingenious 

 that has been suggested, and we hope to see it generally 

 employed. Its great merit consists in enabling the insula- 

 tion of the cable to be observed continually on board, and at 

 the same time, without diselectrifying the cable, to corre- 

 spond. We do not, however, believe that a measuring appa- 

 ratus on shore, where the facilities are so much greater for 

 obtaining exact results, can safely be dispensed with. 



In order that the electrician on board might not always 



DD2 



