1874.] Temperatures of the Sea at various Depths. 467 



40 to 60 lat. there are two insensible currents a lower one towards 

 the equator, and an upper one towards the poles. Arago saw no other 

 explanation than " the existence of submarine currents carrying to the 

 equator the bottom waters of the icy seas." 



We are, however, indebted to Lenz for a full and philosophical review 

 of the whole subject in 1847. After showing that all the facts proved the 

 existence of a temperature of from 34 to 35 F. at depths in the tropical 

 seas, and that this could only be maintained by a constant slow under- 

 current from the poles to the equator (which, on the other hand, must 

 necessitate the transfer by an upper current of the equatorial waters to 

 the poles), he proceeds to show by a series of observations, chiefly those 

 of Kotzebue, and by a diagram, that a belt of cooler water existed at the 

 equator, and that the temperature, at equal depths, was lower there 

 than a few degrees to the north and south of it ; and he concluded 

 that this arose from the circumstance that the deep-seated polar waters 

 there met and rose to the surface. As corroborating this view, he 

 showed that the waters in the same zone were of lower specific gravity, 

 a fact that had been before noticed by Humboldt. 



The author then proceeds to consider some " General Conclusions." 

 Some of these have now been better established by the more recent 

 expeditions and by the researches of Dr. Carpenter. Taking, however, 

 other areas, he shows that in the Arctic Ocean the bathymetrical isotherm 

 of 35 is deepest on the west of Spitsbergen, while nearer Greenland and 

 again nearer Norway the deep waters are colder. The several isothermal 

 planes of 40, 50, 60, 70, and 80 are then traced southward, attaining 

 their maximum depth between 50 and 40 lat., and rising thence towards 

 the equator. Section No. 2, from Baffin's Bay to the equator, shows that 

 the higher isotherms are not prolonged so far north as on the first line, 

 and that the water at the bottom, of the bay is colder than in the Spits- 

 bergen seas, approaching much nearer that of its maximum density and 

 of its point of congelation ; whence he concludes that this is the main 

 source of supply of the deep-seated cold waters in the Atlantic, which, 

 after attaining their greatest depths between latitudes 40 to 50 N., 

 are found 3000 to 4000 feet nearer the surface on approaching the 

 equator. 



In the South Atlantic, the bathymetrical isotherms show lesser curves ; 

 and while the isotherm of 40 crops out between the lat. of 50 and 55, 

 that of 35 is prolonged into high southern latitudes on a nearly uniform 

 plane of 7000 to 8000 feet deep. 



In the Pacific, the sections show that, notwithstanding there is no 

 appreciable polar current through Behring's Straits, the bathymetrical 

 isotherms of 60, 50, and 45 do not extend so far north as in the 

 Atlantic, while that of 35 is apparently not prolonged beyond 60 N. lat. 

 As the presence of temperatures lower than those which prevail in 



