HOW THE SEA IS SOUNDED. 



335 



sea sounding apparatus in which there was attached to the lead, 

 upon the principle of the screw propeller, a small piece of clock- 

 work for registering the number of revolutions made by the little 

 screw during its descent ; and it having been ascertained by ex- 

 periment in shoal water that the appa- 

 ratus in descending would cause the pro- 

 peller to make one revolution for every 

 fathom of perpendicular descent, hands 

 provided with the power of self-regis- 

 tration were attached to the dial, and 

 the instrument was complete. It worked 

 well in moderate depths, but failed in 

 the deep sea on account of the difficul- 

 ty of getting it down if the line used 

 were large enough to give the requisite 

 strength for hauling up. 



Such was the state of the develop- 

 ment of the appliances for measuring 

 the depths of the sea in the middle of 

 the present century, when the idea of 

 using a heavy weight attached to a sim- 

 ple hempen cord was proposed. The 

 plan of stretching a line under the strain 

 of a weight at its lower end from the 

 surface to the bottom underlies the 

 method which is now universally em- 

 ployed for sounding the depths of the 

 sea. In shoal water there is cast from 

 the vessel a plummet in the form of an 

 elongated truncated cone attached to a 

 hempen cord which has been previously 

 divided into feet or fathoms. The line 

 is allowed to run out through the hands of a man who detects, by 

 the sense of touch, the instant when the lead reaches the bottom, 

 and reads the depth by noting the division of the line which cor- 

 responds with the surface of the water. By filling a small cavity 

 in the base of the lead with tallow, a quantity of the sand or gra- 

 vel or mud upon which the lead strikes becomes imbedded in the 

 tallow and gives an indication of the character of the bottom soil. 



The rough surface of a rope presents an obstacle to its free 

 passage through the water, and therefore as the depths increase 

 it is necessary to employ heavier weights to carry the line swiftly 

 in a straight course to the bottom, and, moreover, stronger rope to 

 bear the increased weight of the sinker. In great depths the size 

 of the rope which is necessary is such as to present considerable 

 surface to the action of submarine currents, which carry the line 



FIG. 2. THE SOUNDING CYLINDER 

 WITH SHOT ATTACHED. 



