SCIENCE AND PRACTICE. 399 



^ v TF+F' * M (2 



If we set this value of sin. <p in (1 the expression for k 

 becomes 



whence 



;> _ Q * ... .(3 



In this expression Q and / are constants and known magni- 

 tudes ; therefore, by setting the values 1, 2, 3, &c., in suc- 

 cession for K in (3, we obtain a series indicating the height 

 hj answering to the load Q, when the cable is under the 

 different tensions 1, 2, 3, &c. 



100. The electrical operations during the paying out are 

 of importance. The end of thick cable put ashore is taken 

 into the land-station, and there given in charge of the elec- 

 trician, whose duty it is to see it insulated, to speak through 

 to the ship or to measure. 



Messrs. Siemens, in their expeditions, place a clock at the 

 land station, which puts the end of the cable in position for 

 insulation and continuity tests, and for correspondence, at 

 regular intervals. This arrangement removes the chances 

 of misunderstanding in the event of the insulation becoming 

 bad during the paying out or after it is completed, as the 

 electrician on board the ship knows at what time he may expect 

 to receive currents, and when the farther end is insulated 

 and put to earth. 



The commutator constructed for this purpose consists of a 

 small disc of metal, projections upon the periphery of which 

 come into contact at certain moments in each hour with three 

 metal springs, connected severally to earth, telegraph instru- 

 ment, and measuring board. 



The moment each full hour is completed, the end of the 

 cable, represented by the metal disc just mentioned, makes 

 contact with the Morse-instrument of the telegraphing board, 

 and intelligence can be communicated between the ship and 

 shore through the cable. This contact lasts four minutes, 



