Page:Popular Science Monthly Volume 78.djvu/253

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earth. The four or five thousand separate soundings, made by the Albatross during the past twenty-seven years, indicate a long and persistent bombardment of the ocean depths.

Clamped to the sounding line, a few feet above the bottom specimen cup, is a second brass cylinder used to bring up a sample of the water from the bottom to be tested for its specific gravity. The water bottle is sent down with its valves open, but on being hauled up, the reversed motion of the water against the blades of a propeller-like wheel tightly screws up the valves, and a pint or so of water from the very bottom is brought up through a depth of several hundreds or even thousands of fathoms,

A thermometer in a brass case is also clamped to the sounding line above the water bottle. This is sent down right end up, but on starting back to the surface, a water wheel, similar to that used to close the valves of the water bottle, unscrews a catch holding one end of the thermometer, while the other end is fastened by a loose pin, thus upsetting the instrument, which is now brought to the surface with the mercury in the same position as when it left the bottom. At the same time that the sounding is being made, a specimen of the water at the surface is taken, which is also tested for its temperature and density for comparison with the density of the bottom specimen.

For depths under a hundred fathoms, a hand sounding machine with a light cotton line is used, to which the usual instruments may be attached, unless the navigator wishes merely to determine his position with respect to some shoal. In this case a long weight with a hollow in its end filled with tallow is used, and enough sand or mud sticks to the tallow to indicate the character of the bottom.

On account of cross currents below the surface and on account of the drifting of the ship with the surface currents or the wind, taking a sounding is often extremely difficult, especially if at any great depth, when it requires that the ship be held in one position for several hours. Not infrequently, in spite of the greatest care, the sounding line goes down obliquely instead of perpendicularly and comes across the edge of the rudder or a propeller blade, or a kink is thrown into the line, which causes the wire to snap off at once, and the whole set of instruments is lost. The deepest sounding made from the Albatross, while on her Philippine cruise, was in the Sulu Sea at a depth of over 2,200 fathoms. Several successful dredge hauls have been made at depths of over a thousand fathoms, but most of the work has been done in water less than five hundred fathoms deep among the depressions and along the edges of the partially submerged plateau which forms the Philippine archipelago.

The dredging apparatus carried by the Albatross is of two sorts, the dredges which are dragged over the bottom and the intermediate nets which are trawled between the surface and the bottom. Of the