Geology 



will, of course, weigh more, and so the old difficulty 

 will recur. 



There is also another difficulty to be overcome 

 when far out at sea, out of sight of land, it is impossible 

 to tell whether a ship is stationary or drifting slowly 

 with some surface current. If the ship is drifting the 

 sounding line will trail out astern, for the currents are, 

 as a rule, only near the surface, and thus the sinker 

 will reach the bottom obliquely, and the length of line 

 will no longer be a measure of the depth. Several 

 devices have been invented to overcome these difficulties, 

 of which the following are a few. 



It is often desirable that information should be 

 obtained with regard to the nature of the materials of 

 which the sea floor is made up, and therefore the 

 sounding apparatus is usually constructed in such a 

 manner that it brings up a sample of the bottom for 

 examination. 



In the simple lead which is used by ships approach- 

 ing the land in foggy weather, this object is secured by 

 making the lower end cup shaped, like the bottom of 

 a wine bottle, and, before casting the lead, partially filling 

 the cup with tallow. When the lead reaches the bottom, 

 some of the sand, mud, or other material sticks in the 

 tallow and can afterwards be removed and examined. 

 If no loose material is found in the tallow, it is assumed 

 that there is a rocky or " hard " bottom. 



When greater depths have to be reached, it becomes 

 a matter of some importance to minimize the labour of 

 hauling up the apparatus, and for this purpose several 

 kinds of machine are in use in which the weights are 

 detached automatically on reaching the bottom, and 



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