METHODS OF RESEARCH 15 



inch has recently come into use in the Navy. 

 The stranded wire used with water-bottles 

 and thermometers is about one-eighth of an 

 inch in diameter. 



Trawling and Dredging. — During the " Chal- 

 lenger " Expedition strong hempen lines were 

 used. Sometimes a hempen line eight miles 

 in length was out astern of the ship when 

 trawling in 3000 fathoms. A very great 

 advance was subsequently made when the 

 late Alexander Agassiz introduced wire ropes, 

 about one-third of an inch in diameter, for 

 these operations. The use of piano wire for 

 sounding and steel rope for trawling and 

 dredging is now almost universal. The dredges 

 used on board the " Challenger " (see Plate 

 I.) were 3 to 5 feet in width, and the trawls 

 had beams 10 to 17 feet in length, the smaller 

 size being used in very deep water. On board 

 the " Challenger " a trawling in 3000 fathoms 

 used often to take from 12 to 14 hours. The 

 trawls and dredges are now of many different 

 designs — from the smallest oyster dredge to 

 the large otter trawl with a span of 50 feet. 

 This latter appliance was used successfully 

 by the " Michael Sars " Expedition of 1910 

 in 3000 fathoms (see Plate I.). 



Tow-Nets and Vertical Nets.- — For capturing 

 the animals and plants swimming or floating 

 at the surface and in intermediate waters 



