METHODS OF OCEANOGRAPHICAL RESEARCH 255 



introduction of submarine telegraphy a study of the 

 contours of the bottom at all depths became necessary 

 for the laying of the great transoceanic cables. 



In shallow water the usual method is to heave overboard 

 a lead weight attached to a length of rope which is marked 

 off at intervals with pieces of leather and other materials 

 to indicate the depth in fathoms. The lead weight is 

 usually from ten to fourteen lbs. and has its bottom hollowed 

 out to form a cup into which is put some tallow (Fig. 53). 

 On striking the bottom the bits of sand or pebbles adhere 

 to this grease and the navigator is thus enabled to discover 

 the nature of the bottom over which he is passing. 



Fig. 53. — Sounding lead, showing hollow for " arming " w^ith tallow. 



For sounding in deep water a machine is always used 

 and the sounding line itself is made of thin steel cable, 

 which passes over a grooved wheel each revolution of which 

 corresponds to a known length of wire. In very great 

 depths it becomes impossible to feel when the lead has 

 struck the bottom, because the weight of the many fathoms 

 of wire in use is sufficient to continue unwinding the drum 

 on which it is coiled But by the employment of a brake, 

 however, which can be tightened up to counteract the 

 increasing weight of the wire as it runs off the drum, the 

 machine becomes so delicate that the moment the weight 

 touches the bottom the brake acts and the depth can be 



