542 



Dr. W. B. Carpenter on the 



[June 13, 



be expected to be lower than it is in the Temperate area. And it is a 

 very curious confirmation of this view, that three of the temperatures 

 taken in the ' Mercury ' at 200 fathoms, which ranged several degrees 

 higher than the average, — viz. 60°, 60°, and 58°, — were over a bottom (be- 

 tween Sierra Leone and the Cape-Verde Islands) of which the depth 

 ranged only from 290 to G80 fathoms ; though to the south as well as to 

 the north of these stations, where the depths were respectively 1000 

 and 1100 fathoms, the temperatures at 200 fathoms were 53° and 54°. 

 For if bottom-water, in its gradual rise towards the surface, starts 

 from great depths at an almost glacial temperature, it will obviously be 

 much colder, until it has come under the direct influence of insola- 

 tion, than if it has started from a shallower bottom at a temperature of 

 (say) 45°. 



10. But whilst the Temperature of the upper stratum in our Atlantic 

 sounding does not give any definite indication of the derivation of its water 

 from any remote source, the case is very different in regard to the tempe- 

 rature of the stratum below 1000 fathoms. For I am at a loss to con- 

 ceive how the rapid fall in the Thermometer between 700 and 1000 

 fathoms, and the depression of Temperature below 38° which exhibits 

 itself in this Sounding down to 1560 fathoms, and in the Soundings taken 

 by the ' Porcupine' in 1869 from 1000 fathoms down to 2090 and 2435 

 fathoms, are to be accounted for, unless on the hypothesis that the tempe- 

 rature of this stratum has been reduced by the flow of glacial water from 

 the Polar area towards the Equatorial. For it seems clear that any mass 

 of sea-water which remains long enough on the same bottom, must come to 

 acquire a temperature at least as high as that of the bottom, unless cooled 

 below this either by the action of Atmospheric cold on its surface, or by 

 the intrusion of colder water from some extraneous source. Now, in the 

 case of the Western basin of the Mediteranean, the uniform temperature 

 (54^-°) of the whole stratum beneath 100 fathoms agrees so well on the 

 one hand with the mean temperature of the crust of the earth in that 

 region (Report for 18/0, § 89), and on the other with the lowest winter 

 temperature of its surface*, — while the comparative shallowness of the 

 " ridge" between Capes Spartel and Trafalgar so effectually prevents the 

 admission of any considerable body of water of a temperature below 54^° 

 from the depths of the Atlantic outside, — that we may fairly assume that 

 temperature to be the normal of the Latitude. And since, in the Eastern 

 Atlantic outside the Strait, the whole mass of water below 1000 fathoms 

 has a temperature from sixteen to eighteen degrees below this normal, 

 which cannot be attributed to the action of surface-cold in the locality 

 itself, the inference seems irresistible that this depression must be pro- 

 duced and maintained by the convection of cold from the Polar towards the 



* See the Memoir of M. Aim6 on the Temperature of the Mediterranean, in 1 Ann. 

 de Chim. et de Phys.' for 1845. 



