Page:Popular Science Monthly Volume 44.djvu/348

 more and more out of the vertical direction in proportion to the duration of the passage of the sinker to the bottom, and render the results less and less accurate. Moreover, as the weight of the submerged portion of the rope in addition to the weight of the sinker soon becomes so heavy that a man can not lift it, and therefore can not assure himself by the sense of touch when the lead has reached the bottom; and as the weight of the submerged parts is sufficient at great depths to cause the unwinding of the reel, the line may continue to pass out long after the sinker has reached bottom, and the length unwound may thus bear no relation to the depth to be measured. In addition to these sources of error there is another arising from the drift of the vessel during the period of several hours which is required to effect a deep-sea sounding with rope.

These causes, tending to carry the line off in the direction of the subsurface currents in an ever-increasing complication of loops and bends, and impeding more and more the velocity of the fall of the plummet until it sinks into the oozy soil without communicating to the surface any evidence of its arrival at the bottom, explain the reports of the vast depths of the sea that astonished the public mind less than half a century ago. Lieutenant Berryman, of the United States brig Dolphin, reported an unsuccessful attempt to fathom mid-ocean with a line thirty-nine thousand feet in length. Captain Denham, of her Britannic Majesty's ship Herald, reported bottom in the South Atlantic at a depth of forty-six thousand feet; and Lieutenant J. P. Parker, of the United States frigate Congress, in attempting to sound the same region, let go his plummet and saw fifty thousand feet of line run out after it as though the bottom had not been reached. The deepest spot in the South Atlantic is not more than twenty thousand feet beneath the rolling waves that sealed its mysteries fifty years ago; and the deepest spot yet discovered in the world not more than twenty-eight thousand feet.

By the use of wire for sounding great depths many of the difficulties and uncertainties which characterize the use of rope are obviated, for the wire, being light in weight and of small cross-section, is not greatly affected by submarine currents, but allows