18G8.] 



on Deep-sea Dredgings. 



187 



existence of a stratum of sea-water at a temperature of 32° or even 28° 

 below a stratum at 39°, is evident from the fact (which has been experimen- 

 tally established beyond question *) that Sea-water, in virtue of its saline 

 impregnation, contracts continuously down to its ordinary freezing-point, 

 which is below 28° Fahr. And the existence of such strata, even in 

 Equatorial regions, has been regarded by high scientific authorities f as 

 proving the existence of deep currents bringing cold water from Polar Regions 

 to replace the warmer water that is continually flowing, as (notably) in the 

 Gulf-stream, from the Equatorial towards the Polar Regions, as well as to 

 make good the immense loss which is constantly taking place by evapo- 

 ration from the surface of Tropical seas %. To such an under-current, pro- 

 bably proceeding from the North or North-east, the low temperatures we 



Journal of Capt. Ross's Voyage, which, if there was no error in the instrument employed, 

 gives a lower temperature than any yet recorded : — " Having sounded, on Sept. 19, 

 1818, in 750 fathoms, the registering Thermometer was sent down to 680 fathoms ; 

 and on coming up, the index of greatest cold was at 25 0, 75. Never having known it 

 lower than 28° in former instances (even at a depth of 1000 fathoms, and at other 

 times when close to the bottom), I was very careful in examining the Thermometer; 

 but could discover no other reason for it than the actual coldness of the water." 



* It is stated by M. Despretz, as the result of a series of carefully conducted experi- 

 ments, that the maximum density of Sea-water cooled down continuously without agita- 

 tion is at — 3 0, 67 Cent., or 25°-4 Fahr. ; the freezing-point of Sea-water which is agitated 

 being — 2°'55 Cent., or 27°*4 Fahr. See his " Recherches sur le Maximum de Densite des 

 Dissolutions Aqueuses," in Annales de Chimie, 1833, torn. lxx. p. 54. 



f This doctrine, long since explicitly stated by Humboldt (Cosmos, vol. i. p. 296), is 

 thus set forth by Prof. Buff in his 1 Physics of the Earth ' (p. 194) :— « The water of 

 the ocean at great depths has a temperature, even under the equator, nearly approach- 

 ing to the freezing-point. This low temperature cannot depend on any influence of 



the sea-bottom The fact, however, is explained by a continual current of 



cold water flowing from the Polar regions towards the Equator. The following well- 

 known experiment clearly illustrates the manner of this movement. A glass vessel is to 

 be filled with water with which some powder has been mixed, and is then to be heated 

 at bottom. It will soon be seen, from the motion of the particles of powder, that cur- 

 rents are set up in opposite directions through the water. "Warm water rises from the 

 bottom, up through the middle of the vessel, and spreads over the surface, while the 

 colder and therefore heavier liquid falls down at the sides of the glass. Currents like 

 these must arise in all water-basins, and even in the oceans, if different parts of their 

 surface are unequally heated. The water that is cooled in the polar regions sinks and 

 travels from the poles towards the equator, pushing away the warmer and lighter liquid 

 from the bottom of the sea ; itself to give way in turn, as it gets warm, to the colder 

 water that follows after it. This continual flow of the water from the cold zones is re- 

 placed in a twofold manner. The w r arm water of the tropical seas, since it is the lightest, 

 must spread itself north and south over the surface of the ocean, and thus gradually losing 

 its heat, be carried to the polar regions. Between the tropics, too, evaporation goes on 

 most vigorously, and a great part of the vapours formed fall again in rain and snow only 

 in higher latitudes." 



% A set of deep soundings, taken across the Arabian Sea, between Aden and Bombay, 

 by Capt. Shoiiland, in H.M.S. ' Hydra,' has lately been received by the Hydrographer 

 to the Admiralty, which give a line of bottom-temperature of 33J° [0 o, S Cent.] at depths 



VOL. XVII. P 



