TRANSACTIONS OF THE SECTIONS. 185 



On the Temperature obtained In the Atlantic Ocean during the Cruise of H. M.S. 

 ' Challenger.' By Staff-Commander Tizaeb, B.N. 



Over a gveat portion of the Atlantic the bottom temperature has this peculiarity: 

 — If the depth be less than 2000 fathoms, we find tlie temperature at tlie bottom 

 lower than that of any intermediate depth ; but when the depth exceeds 2000 

 fathoms, we find that the bottom temperatures are nearly the same as they are at 

 that depth. This holds good for three fourths of this ocean. In the remaining 

 fourth the temperature obtained at the bottom is much lower than in the other 

 parts ; and this fourth is not at cither extreme, where there is a large current of 

 surface cold, but occupies the whole of the western portion of the South Atlantic 

 as far north as tlie Equator. The residts of these temperatures may be classihed 

 thus : — If an imaginary line be drawn from French Guiana to tlie westernmost island 

 of the Azores, and from thence nortli on tlie westeim side of this line, the bottom 

 temperatures at depths exceeding 2000 fathoms are 35° ; that is, taking the mean 

 of all the temperatures obtained, which differ but slightly. On the eastern side of 

 this line the bottom temperatures are 35°'3 ; and tliis imiform temperature appears 

 to extend as far south as Tristan d'Acimha, as the German frigate ' Gazelle ' ob- 

 tained similar bottom temperatures eastward of the line joining that island with 

 Ascension to the southward of a line "joining Tristan d'Acimha with the Cape of 

 Good Hope. The bottom temperatures are decidedly colder between the eastern 

 coast of South America and a line joining Tristan d'Acuuha and Ascension Island ; 

 and from the Equator to the southward the bottom temperatures were invariably 

 colder than at any intermediate depth. These temperatures varied from 31° to 

 33°'5, that i^, when the depth exceeds 2000 fathoms ; and temperatures of less 

 than 33° were found as far north as the Equator, while a few miles northward 

 this bottom temperature was 3o°. It therefore appears that in the western portion 

 of the South Atlantic the highest bottom temperature is less than the lowest 

 obtained elsewhere in this ocean, excepting where tlie very low result of 29° was 

 found by the ' Porcupine ' in 18G9 between the Faroe Isles and the north extreme 

 of Scotland. The question thus arises as to the causes which confines this cold 

 water to the bottom portion of the western half of the South Atlantic. The 

 examination of the soundings which had been taken in this ocean, combined with 

 the results of their temperature, leads to the conclusion that there is a series of 

 ridges dividing its bed into two basins, one of which occupies the whole of the 

 western portion of the North Atlantic, while the other extends the whole of the 

 length of the ocean on its eastern side, and that the cold water in the western 

 ■portion of the South Atlantic is owing to there being no obstruction between the 

 bed of this portion of the ocean and the Ijed of the Antarctic basin ; and from the 

 results of the serial temperatures' soundings it would appear that these ridges can- 

 not exceed 1850 or 2000 fathoms in depth. To ascertain the thermal condition cf 

 the Atlantic ffrom the surface to the bottom), serial temperatures were obtained 

 iu the ' Challenger' at 150 positions, observations having been made at each ICO 

 fathoms to 1500 fathoms iu depth, and frequently at, sa}-, 10 fathoms to 200 fathoms 

 in depth, at each of these positions. An examination of these temperatures shows 

 that between the parallels of 40° N. and 40° S. there is a much larger amount of 

 warm water in the North than in the South Atlantic, and that in the equatorial 

 regions the isotherm of 00° is much nearer the surface tlian in the temperate zones, 

 but that the isotherms below G0° are at nearly as great a depth at the Equator as 

 in any part of the South Atlantic, especially at the isotherm of 40°, and that 

 between the ])arallel of 30° and 40° N. latitude the isotherm of 60° occupies a 

 depth of 300 fathoms over an area of 1,200,000 square miles, while the average 

 depth of this isotherm between the parallels of 30° and 40° S. latitude is IGO fathoms ; 

 also that the isotherm of 40°, which is at an average depth of 800 fatlioms across 

 the North Atlantic, between the parallels of 30° and 40° N. latitude, occupies only 

 half that depth in any part of the South Atlantic. This phenomenon may be ex- 

 plained in the following manner : — The power oi the sun indirectly heating the water 

 below the surface appears not to extend below 100 fathoms even in tlie tropics ; 

 and this power decreases as the liigher latitudes are reached, mitil a position 

 is attained where the temperature is that of the freezing-point of salt water. As 



