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HOW THE SEA-DEPTHS ARE EXPLORED.
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The method of dredging at a great distance is thus graphically described by Prof. Thomson, as it was performed in the Bay of Biscay, July 22, 1869. The depth was first accurately ascertained by sounding
Fig. 7.
The Stern Derrick of the Porcupine, showing the "Accumulator," the Dredge, and the Mode of stowing the Rope.
and found to be 14,610 feet. "At 4.45 p. m. the dredge was let go, the vessel drifting slowly before a moderate breeze from the north--
