DEEP-SEA SOUNDING. 139 



line, some with spun-yarn, and some with a slender 

 thread of silk ; but all had proceeded upon the assump- 

 tion that, as soon as the weight touched the bottom, 

 either the shock would be perceptible to the hand, or 

 the line would instantly slacken, and cease to run off 

 the reel. 



These assumptions were, however, fallacious. It is 

 found that the diminution of weight, caused by the 

 resting of the lead, when vast lengths of the line are 

 out, is not perceptible to the human hand ; and, more- 

 over, that there are currents in the profundities of the 

 sea, which belly-out and carry away the line long after 

 the plummet is at rest ; and this even when, owing to 

 the freedom from current of the superficial strata, the 

 line appears to be perpendicular. Thus immense 

 lengths of line were run out, but no satisfactory 

 soundings were obtained. 



Then other devices were projected. One thought 

 that a charge of powder, in a sort of shell, might be 

 exploded by the shock of striking the bottom ; and 

 that the reverberation being heard at the surface, a 

 judgment might be formed of the depth, from the 

 rate at which sound is known to travel through water. 

 But the experiment did not answer expectation. The 

 shell exploded, but the surface gave no sign. Sound- 

 ing-plummets were constructed, having a column of 

 air within them, which would indicate the amount of 

 pressure to which it had been subjected. In mode- 

 rate depths, these answered well ; but, in great deeps, 

 just when their aid was wanted, they failed ; for the 



