PHYSICS OF THE GLOBE. 113 



Belknap's modification of the Thompson machine, with Bel- 

 knap's detaching rods and specimen cups, and the Miller- 

 Casella deep-sea thermometer for temperatures. This line 

 furnishes an admirable cross-section of the South Atlantic, 

 defining the limits of the two great longitudinal deep chan- 

 nels, and of the bank or ridge between them, on which latter 

 are located the islands of Tristan d'Acunha, St. Helena, and 

 Ascension. The soundings were taken, as a rule, about 100 

 miles apart. After leaving the coast of Africa, the depths 

 increased very rapidly, 900 fathoms being reached within 

 60 miles of the initial point; 2200 fathoms were reached 

 in lat. 9 40' S. and long. 10 36' E. ; 25S1 fathoms in lat. 

 10 12' S. and long. 8 57' E. ; 3000 fathoms in lat. 11 20' 

 S., long. 5 34' E., 700 miles from St. Paul. The greatest 

 depth found in the Eastern Channel was 3063 fathoms. The 

 3000-fathom curve on the western side of the channel was 

 passed in lat. 13 30' S., long. 10 W. Thence the depths 

 gradually decreased until within 7 miles of the Sugar Loaf, 

 St. Helena, where the depth was 2091 fathoms. Within the 

 same distance to the westward of St. Helena the depths 

 were scarcely less great, showing the remarkable fact that 

 the island is the head of a pinnacle standing in 2000 fathoms 

 of water. Continuing, a depth of 2333 fathoms was found in 

 lat. 16 52' S., long. 9 53' W.; thence a gradual diminution, 

 until in lat. 17 57' S., long. 15 17' W., a depth of but 1365 

 fathoms was found. This point is probably not far from the 

 crest of the divide above spoken of, between the two chan- 

 nels, or, as they might better be described, the two great 

 submarine valleys. It is to be regretted that the soundings 

 in this vicinity had not been taken much nearer together, 

 as a less depth might have been discovered. Sir George 

 Nares, who ran, in the Challenger, a line along the crest of 

 the ridge approximately, found at a point 90 miles to the 

 northward and eastward of this a depth of 1415 fathoms. 

 Proceeding, the depths increased rapidly from the last sound- 

 ing: 2000 fathoms were reached in lat, 18 15' S., long. 16 

 59' W., 100 miles from St, Helena; 2652 fathoms in lat. 18 

 48' S., long. 19 21' W. ; 3284 fathoms, the greatest depth 

 found in the Western Channel, in lat. 19 55' S., long. 24 50' 

 W. The 2500 -fathom curve on the western slope of the 

 channel was passed in lat. 21 10' S., long. 32 W.; the 2000- 



