238 THE VOYAGE OF H.M.S. CHALLENGER. 



indications of the coldest water being at the greatest depth, were not so marked as 

 between St. Paul's Rocks and the American coast, for the temperature of 34°"2 was 

 found at the depth of 2150 fathoms, whilst at 2350 fathoms there was a temperature 

 of 34°'7. The former temperature, however, rests on the indication of one thermometer, 

 whilst the latter is the mean of two, and the thermometer that gave the temperature 

 of 34°'2 at 2150 fathoms gave a temperature of 34 0, 4 at 2350 fathoms. It is probable, 

 therefore, that the temperature of 34° 2 may be 0°*5 too low. 



The serial temperature soundings showed that, notwithstanding a change of surface 

 temperature of 24°, and of latitude of 18°, the isotherm of 40° was nearly parallel with 

 the surface, its average depth being 410 fathoms, and its range 80 fathoms, viz., from 

 380 to 460 fathoms. Above the isotherm of 40° the temperature increased gradually to 

 the surface (see Diagram 5). 



No observations on currents were made, except the ordinary ones of ascertaining 

 its direction and strength, by means of the difference of the position of the ship, by 

 observation and dead reckoning. 



On the 3rd October, at Station 130, the velocity of the wind was 18 miles per hour in 

 the forenoon and 20 in the afternoon, the force registered being 4 ; on the 6th October, at 

 Station 131, the velocity was 12 miles per hour, the force registered being 3 ; and on the 

 14th October, at Station 134, the velocity was 11 miles per hour, the force registered 

 being 2. On the 15th October, at anchor off Tristan daCunha, the velocity was 16 miles 

 per hour, the force registered 4 to 5. 



On the 14th October, at daylight, the peak of Tristan da Cunha was sighted, bearing 

 S.S.W., distant 54 miles, and at 7 a.m. the ship stopped to sound and dredge at Station 

 134 (see Sheet 16). At 3 p.m., having completed dredging, a course was shaped for 

 the island, the weather being fine; and at 10 p.m. the vessel was stopped off it, and 

 "laid to" for the night. The peak of Tristan was visible in the early morning, but 

 clouded over at 8 a.m., and was not seen during the remainder of the day, except for a 

 short interval at 5 p.m. 



Between the coast of America and Tristan the greatest depth obtained was 2350 

 fathoms. There were many indications of an extensive plateau surrounding the Tristan 

 group, with depths varying from 1425 to 2000 fathoms. 



The deposits in depths less than 2100 fathoms on the Tristan plateau contained from 

 85 to 95 per cent, of carbonate of lime, which was almost wholly composed of the shells of 

 pelagic organisms, whilst the three soundings in depths greater than 2100 fathoms towards 

 the American coast contained from 35 to 55 per cent. It was observed that as the 

 ship proceeded southward the Foraminifera in the deposits became dwarfed, and some 

 tropical species disappeared. There were quartz fragments in the deposits near the 

 American shores, but these disappeared or were exceedingly rare in the deposits towards 

 the centre of the South Atlantic. 



