DEEP-SEA SOUNDINGS. 25 



from the fact that, after a large quantity of line 

 has been run out, the shock of the lead striking 

 the bottom cannot be felt. Moreover, there is 

 sufficient force in the deep-sea currents to sweep 

 out the line after the lead has reached the 

 bottom ; so that, with the ordinary sounding-lines 

 in use among navigators, it is impossible to sound 

 great depths. Scientific men have, therefore, taxed 

 their brains to invent instruments for sounding 

 the deep sea for touching the bottom in what 

 sailors call " blue water." Some have tried it with 

 a silk thread as a plumb-line, some with spun-yarn 

 threads, and various other materials and contriv- 

 ances. It has even been* tried by exploding petards 

 and ringing bells in the deep sea, when it was 

 supposed that an echo or reverberation might be 

 heard, and, from the known rate at which sound 

 travels through water, the depth might thus be 

 ascertained. Deep-sea leads have been constructed 

 having a column of air in them, which, by com- 

 pression; would show the aqueous pressure to which 

 they had been subjected ; but the trial proved to 

 be more than the instrument could stand. 



Captain Maury, of the -American Navy whose 

 interesting book has been already referred to 

 invented an instrument for sounding the deep sea. 

 Here is his own description of it : " To the lead 

 was attached, upon the principle of the screw- 

 propeller, a small piece of clock-work for registering 

 the number of revolutions made by the little screw 



