THE CONQUEST OF TIME AND SPACE 



piece of mechanism calculated to test the depth of the 

 waters in which he finds himself at any given moment. 

 In its most primitive form — in which form, by the bye, 

 it is still almost universally employed — this apparatus 

 is called the lead, — so called with much propriety be- 

 cause it consists essentially of a lump of lead or other 

 heavy weight attached to a small rope. Knots in the 

 rope enable the sailor who manipulates the lead to note 

 at a glance the depth to which it sinks. Most ocean 

 travelers have seen a sailor heaving the lead repeatedly 

 at the side of the ship and noting the depth of the 

 water, particularly as the ship approached the Long 

 Island shore. 



While this simple form of lead suffices for ordinary 

 purposes, when the chief information sought is as to 

 whether the water is deeper than the draft of the ship, 

 it is at best only a rough and ready means of testing 

 the depth in relatively shallow waters. For deeper 

 waters and to test with greater accuracy the depths of 

 uncharted regions, and in particular to determine the 

 character of the sea bottom at any given place, more 

 elaborate apparatuses are employed. One of the most 

 useful of these is the invention of the late Lord Kelvin. 

 In this the lead is replaced by a cannon ball, perforated 

 and containing a cylinder which is detached when the 

 weight reaches the bottom and is drawn to the surface 

 filled with sand or mud, the cannon ball remaining at 

 the bottom. In another form of patent lead, a float 

 becomes detached so soon as the weight strikes the 

 bottom and comes at once to the surface, thus record- 

 ing the fact that the bottom has been reached, — a fact 



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