HOW THE SEA-DEPTHS ARE EXPLORED. 267 



The method of dredging at a great distance is thus graphically de- 

 scribed by Prof. Thomson, as it was performed in the Bay of Biscay, 

 July 22, 1869. The depth was first accurately ascertained by sound- 



FlG. 7. 



Tire Stern Derrick op the Porcupine, showing the " Accdthulator," the Dredge, and the 



Mode of stowing the Rope. 



ing, 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- 



