628 Prof. "W. Thomson on the Voyage of the ( Challenger ' 



central rise, with an average depth of 1850 fathoms. The mean bottom- 

 temperature of the seven deep soundings is — 0°-4 C, and that of the five 

 soundings on the rise 1°'3 C. ; the isothermobath of o, 0. is at a depth 

 averaging 2400 fathoms, a depth which it never much exceeds except 

 where the cold water rises against the American coast, as at Stations 319 

 and 323 ; it therefore occurs in the line of the seven deep soundings only; 

 and there it forms the upper limit of a mass of water with a temperature 

 below zero, 320 square miles in section. Perhaps the isothermobath of 

 l°-5 C. may fairly be taken as the upper limit of the very cold water ; 

 the section of the Antarctic indraught below that temperature is here 

 about 800 square miles. (The transverse -section of the Gulf-stream is 

 about 6 square miles ; there is no volume of water at all in the Labrador 

 current below 1°*5 C. opposite Halifax, that temperature being only 

 found at the bottom.) 



The isothermobaths of 2°, 2°*5, 3°, and 4° 0. are very constant at 

 1500, 900, 600, and 400 fathoms respectively for all the stations on the 

 parallel except Station 323 on the " cold wall," where all the lower tem- 

 perature-lines are at a much higher level, and at the shallow sounding 

 at Station 331, where all the lines below that of 4° C. rise slightly. "We 

 must be careful, however, not to attach too much importance to slight 

 deviations of the colder lines. On the scale used in Plates 26, 28, 32, 

 and 33, the mean interval between the isothermobaths of 2° and 3° C, 

 in the Atlantic is 1000 fathoms ; so that a rise or fall of 100 fathoms, 

 which is very prominent on such diagrams, actually represents only 

 one tenth of a Centigrade degree, an amount very small in itself, and, 

 it must be remembered, entirely within the limit of error of obser- 

 vation with a deep-sea thermometer ; it is only where there is a con- 

 cordance among several lines in such a rise or fall that the indication is 

 of any real value. 



The mean temperature of the surface of the water at the anchorage 

 at Monte Video was 22°*5 C. ; at Station 320, just before entering the 

 estuary, it was 19 0, 7 C. ; at Station 323, on the edge of the plateau, it rose 

 to 23° ; and a temperature of 20° 0. or a little above it was maintained 

 up to Station 328, except at Station 326, which I have already referred 

 to as occupying a cold space. After reaching Station 328 the tempera- 

 ture began to sink, and only recovered its former height when we moved 

 northwards. 



The curves deduced from the serial temperature-soundings along this line 

 are represented in groups of four on Plates 29, 30, & 31. At Station 323 

 the temperature of the water fell rapidly and tolerably evenly, with only 

 a slight prominence between 150 and 250 fathoms, to 5° C. at 350 

 fathoms ; it then took a slower, but still a symmetrical sweep down to 

 zero at 1900 fathoms. At Stations 324 and 325 the hues fell, with 'still 

 only a slight protuberance in the same position as before, to 3° C. at 600 

 fathoms, when they sank very slowly to 1° 0. at 2000 fathoms, and then 



