§ 5G9. THE DEPTHS OF THE OCEAN. 311 



reels of ten thousand fathoms each. It is made the duty of the 

 commander to avail himself of every favorable opportunity to 

 try the depth of the ocean, whenever he may find himself out 

 upon '• blue water." For this purpose he is to use a cannon ball 

 of 82 or 68 pounds as a plummet. Having one end of the twine 

 attached to it, the cannon ball is to be thrown overboard from a 

 boat, and suffered to take the twine from the reel as fast as it will. 

 The reel is made to turn easily. A silk thread, or the common 

 wrapping-twine of the shops, would, it was thought, be strong 

 enough for this purpose, for it was supposed there would be no 

 strain upon the line except the very slight one required to drag 

 it down, and the twine having nearly the specific gravity of sea 

 water, this strain would, it was imagined, be very slight. More- 

 over, when the shot reached the bottom, the line, it was thought 

 (§ 561), would cease to run out ; then breaking it off, and seeing 

 how much remained upon the reel, the depth of the sea could be 

 ascertained at any place and time simply at the expense of one 

 cannon ball and a few pounds of common twine. 



569. But practical difficulties that were not expected at all 

 Discovery of currents wcrc lurkiug in the way, and afterward showed 



in the depths of the , ^ ^ 



sea. themselves at every attempt to sound ; and it was 



before these practical difficulties had been fairly overcome that 

 the great soundings (§ 567) were reported. . In the first place, it 

 was discovered that the line, once started and dragged down into 

 the depths of the ocean, never would cease to run out (§ 562), 

 and, consequently, that there was no means of knowing when, if 

 ever, the shot had reached the bottom. And, in the next place, 

 it was ascertained that the ordinary twine (§ 566) would not do ; 

 that the sounding-line, in going down, was really subjected to 

 quite a heavy strain, and that, consequently, the twine to be used 

 must be strong; it was therefore subjected to a test which re- 

 quired it to bear a weight of at least sixty pounds freely suspend- 

 ed in the air. So we had to go to work anew, and make several 

 hundred thousand fathoms of sounding-twine especially for the 

 purpose. It was small, and stood the test required, a pound of it 

 measuring about six hundred feet in length. The officers in- 

 trusted with the duty soon found that the soundings could not 

 be made from the vessel with any certainty as to the depth. It 

 was necessary that a boat should be lowered, and the trial be made 



