The Hydrosphere 



only the inner part, consisting of a metal tube provided 

 with a valve at its lower end, so as to retain a sample 

 of the bottom, is again brought to the surface. In this 

 form of apparatus a strong wire is used in place of the 

 cord or rope, and being thinner and therefore offering 

 less surface to the water, there is less tendency for it 

 to trail out obliquely or be deflected by currents. 



When dealing with very great depths even these 

 forms of apparatus were found to be unreliable, as it 

 was impossible to ascertain if the machine had gone 

 straight down, and therefore an appliance which would 

 tell the depth independently of the length of line used 

 had to be devised. 



The late Lord Kelvin invented a most ingenious 

 form consisting of a strong glass tube closed at one 

 end and similar to those in use in barometers. The 

 tube was coated on the inside with chromate of silver, 

 a reddish - brown substance, and was placed in the 

 sounding machine with its closed end uppermost. When 

 this was lowered into the sea, the water would not at 

 first enter the tube far beyond its mouth, but as it 

 sank deeper and deeper the pressure would gradually 

 increase and the water would be forced farther and 

 farther up the tube, gradually compressing the air in 

 the closed end. 



Now, sea water, as everyone knows, contains common 

 salt, or chloride of sodium, and this acts upon the 

 chromate of silver in the tube, forming chloride of silver, 

 which is white. 



As the machine, having dropped its heavy sinkers 

 at the bottom, is hauled up, the sea water will again be 

 forced out of the tube by the bubble of compressed 



43 



