190 



SUBMARINE CABLE LAYING AND EEPAIRING. 



slot in the disc, preventing it from being raised too iiigh and 

 decentralising the mirror. The whole, when completed and 

 adjusted, is slipped into a brass tube, B, provided with a glass 

 window at the end near the mirror, and filled with water. The 

 tube is then slipped into the coil of the instrument. 



The tube must be completely immersed when filling and 

 screwed up while immersed to exclude all air, and a vent must 

 be provided to let out excess water and allow for subsequent 

 expansion. 



The water-damped mirror illustrated in Fig. 97a has been 

 devised by Mr. F. Jacobs to get an effectual water seal and 

 space for expansion with a perfectly tight tube. 





Fig. 97a.— Jacobs' Dead-beat Mirror. 



The screwed plug N and centre stem S are in one, and when 

 the mirror is suspended ready for use the tube is filled, or 

 nearly so, with water and the plug screwed in. Some air is 

 intentionally left in the tube and this rises to the top of the 

 enlarged annular space at the end. It will be seen therefore 

 that the air cannot get to the mirror part of the tube, and its 

 presence allows room for expansion of the liquid without the pro- 

 vision for an external vent. The tube can therefore be com- 

 pletely closed in. This form of tube has proved itself of great 

 service in ship-work and is used by Messrs. Siemens Brothers 

 &Co. 



It is remarkable that the mirror instrument is so sensitive 

 to cable signals when the suspension fibre is fastened top and 

 bottom and there is only about one-sixteenth of an inch of fibre 

 above and below the mirror free to turn. The first experi- 



