1872.] 



' Shearwater 3 Scientific Researches. 



541 



bottom-soundings taken in the ' Porcupine ' along the coast of Spain and 

 Portugal, as to the well-marked distinction between the upper warm 

 stratum and the lower cold stratum, with a stratum of intermixture be- 

 tween the two. For whilst the reduction of temperature beneath the 

 superheated stratum of the surface was only from 57°* 7 at 100 fathoms 

 to 50 o, 5 at 700 fathoms — that is to say, 7°'2 in 600 fathoms, or at the 

 rate of 1 0, 2 per 100 fathoms, — the reduction between 700 fathoms and 

 1000 fathoms was 12°'5, or at the rate of nearly 4 0, 2 per 100 fathoms ; 

 while the whole mass of the deeper water from 1000 fathoms down to the 

 bottom at 1560 fathoms was found to have a temperature between 38° 

 and 37i°. 



7. The significance of these facts becomes most apparent when they are 

 graphically contrasted (as in Plate III.) with the temperature-phenomena 

 of the Mediterranean under nearly the same parallel ; for, as was proved 

 by the * Porcupine' Temperature- soundings of 1870 in the Western basin, 

 the temperature from 100 fathoms (at which depth the influence of direct 

 insolation appeared to cease) down to the bottom at 1400 or 1500 fathoms 

 is uniform throughout *, namely about 54 |°. It is clear, therefore, (1) 

 that depth per se does not give rise to any change in the temperature of 

 sea- water occupying a basin which has only a superficial connexion with 

 the great Ocean-system ; and consequently (2) that the diversities of tem- 

 perature shown in the different strata of the Atlantic must depend upon 

 their derivation from different sources. 



8. Now with regard to the upper stratum lying between 100 and 700 

 fathoms, there is no evidence of its derivation from any Southern source ; 

 on the contrary, the Temperatures taken at 200 and 400 fathoms, in the 

 recent voyage of the New York School-ship 'Mercury* from Sierra Leone 

 to Barbadoes (the Report of which has been kindly transmitted to me by 

 Prof. Draper), indicate that below the stratum superheated by direct inso- 

 lation, the temperature is even lower between the Tropics than it is in this 

 more northern portion of the Atlantic area. For the average of nine 

 observations at 200 fathoms, taken where the depth of the Oceanic basin 

 was 1000 fathoms or more, was almost exactly 52°, the range being only 

 between 54° and 50° ; and the temperature at 400 fathoms, about 350 

 miles east of Barbadoes, was only 47°. 



9. If this last fact should be confirmed by further inquiries, it will 

 furnish a powerful argument in support of the doctrine of a vertical 

 Oceanic Circulation which I have repeatedly advocated ; since it is obvious 

 that as it will be in the Intertropical area that the cold bottom-water will 

 be most rapidly brought up to the surface (Report for 1870, § 127), the 

 temperature of the stratum not yet within reach of direct insolation might 



* This uniformity had been previously observed by Capt. Spratt ; although, from the 

 want of " protection " of his Thermometers, he had assigned too high a temperature to 

 the deeper stratum of the Mediterranean basin. 



