ON DEEP-SEA SOUNDING. 381 



mcnts, and methodical practice, as part of regular 

 naval drill in the use of the sounding machine, one 

 minute of time, or from that to four minutes, suffices 

 to take a sounding. 



A description of the machine and rules for its 

 use are given in my printed paper of instructions. I 

 have only now to add a few words regarding the 

 depth gauge. Erichsen's self-registering sounding 

 lead (patented in 1836) depending on the compres- 

 sion of air might be used with my machine, but the 

 simpler form before you is preferable as being surer. 

 It too depends on the compression of air, but in it 

 the extent to which the air has been compressed is 

 marked directly on the interior of a straight glass 

 tube by the chemical action of sea water on a 

 preparation of chromate of silver with which the 

 tube is lined internally. Between the salt of the 

 sea water and the chromate of silver a double 

 decomposition takes place. The chlorine leaves the 

 sodium of the common salt and combines with the 

 silver, while the chromic acid and oxygen leave the 

 silver and combine with the sodium. Thus chloride 

 of silver, white and insoluble, remains on the glass 

 in place of the orange-coloured chromate of silver 

 lining as far up as the water has been forced into 

 the tube, and the chromate of sodium dissolved in 



there can be clanger of shipwreck by running on rocks or on shore. 

 "\VHKN NEAR SHORE OR WITHIN 100 FATHOMS IN THICK 

 WEATHER KEEP THE MACHINE GOING INCESSANTLY, TWO MEN- 

 WORKING IT AND ONE TO RELIEVE." 



