8o THE SCIENTIFIC PAPERS OP 



varied it was a sure indication that there was some defect, which , 

 after the cable was laid, would probably develop itself into a fault. 

 But the value of these tests extended much further, if either during 

 the laying of the cable, or afterwards, any slight decrease of 

 insulation occurred, it would at once show the existence of a slight 

 fault, although the line, if measured by others unacquainted with 

 the previous tests, might appear perfect. The position of that 

 fault should be immediately determined, and be carefully watched 

 from day to day. In fact complete records of the condition of the 

 cable ought to be telegraphed each day, or each second day, to the 

 chief superintendent of the line, in order that he might be able to 

 direct timely operations of repairs. So long as there was a single 

 fault in the line, they could by their methods of testing, find out 

 its position with the greatest certainty. 



It had been originally intended that the Eangoon cable should 

 be immersed in water during its entire progress. After having 

 been tested at the gutta-percha works it was to have been placed 

 at the contractor's works in tanks, leaving them only for the short 

 space of time necessary for passing the cable through the machine. 

 From these tanks it was eventually to have been coiled into others 

 on board the ships, in order that it might be payed out from water 

 into the sea. But the tanks of the contractor were unable to 

 support the great pressure of water, and thus the cable became 

 exposed to atmospheric influences. It was soon observed that 

 there was a loss of insulation, indicating an increase of tempera- 

 ture, which eventually became so great that mist was seen to 

 arise from one portion of the cable and it became necessary to pour 

 water over it. Thereupon the Government requested Professor 

 Miller to investigate the subject chemically, and they called upon 

 Mr. Siemens to make a report on the electro-thermal phenomena. 

 It was requisite for this purpose to test the temperature of every 

 part of the coil, for which Mr. Siemens devised a peculiar ther- 

 mometer constructed upon the principle of the resistance of copper 

 wire to the electric current, varying, in a fixed ratio, with the 

 changes of temperature. It consisted of a rod or tube of metal, 

 round which were wound several layers of fine wire covered with 

 silk ; and the whole was hermetically sealed with india-rubber 

 and gutta-percha to prevent the access of the water. The two 

 ends of the wire were then brought in contact with the instrument 



