GULF STREAM— TEMPEEATUEE. 



9 



Fig. 4. — Tempeuatuke 

 OF THE Ocean West 



OF ROCKALL. 



According to Sir Wyville 

 Thomson. 



TEMP. -^ 

 57 2 I 



53'S ^^E 

 500 I 



and so dense that one end of the vessel was invisible at the other. During the 

 month of June they never once beheld a blue patch of sky. On the other hand, 

 the atmosphere in these seas is generally calm, and the storms are seldom very 

 fierce, although the low temperature makes them at times seem more violent than 

 they really are. Most of them are of short duration, and all end invariably in 

 absolute stillness.* • 



The temperature of the surface waters has enabled meteorologists to determine 

 the outward limits of the North Atlantic warm current. Thermometrical 

 calculations made in the deep waters have also revealed the 

 normal depth of the current in the various seas that have 

 been scientifically explored. But such delicate and costly 

 observations have hitherto been necessarily restricted to a very 

 small portion of the oceanic area. Till quite recently our 

 information on the subject was limited to the revelations of 

 Wyville Thomson and Carpenter, aided by other naturalists 

 who took part in the explorations of the LigJitning and 

 Porcupine in 1868 and 1869. Since then these seas have 

 been again explored under the direction of Swedish and 

 Norwegian scientists, and in 1877 nearly the whole of the 

 Norwegian waters were visited by the meteorologist Mohn 

 in the Von'ngen, who has thus been enabled to draw up an 

 isothermal chart based on his own observations and those of 

 his predecessors. t 



The observations having been made during the fine season, 

 when the surface waters are exceptionally heated by the 

 rays of the summer sun, the hitherto observed surface 

 temperature was always high, falling rapidly in the deeper 

 strata for about 5o fathoms. But the reverse was found to 

 be the case in winter, when the surface was cool. The 

 temperature was then observed to rise to a stratum of normal 

 heat indicating the mean of the year, and found at a depth of 

 not less than 55 fathoms. But at this point the local climatic 

 influences cease, and the plummet penetrates the ocean depths 

 in a temperature unaflfected by the sudden changes of the 

 seasons. Below the zone chanmng- from winter to summer, 

 the thermometer indicates a steady diminution of heat, the strata growing colder and 

 colder without any reaction whatsoever. The lowest temperature thus corresponds 

 with the lowest depth ; yet it was nowhere found to reach the freezing point, which 

 for sea- water with a mean saline density is 25° 4' Fahr.+ The thermometrical 

 soundings of Sir Wyville Thomson and his associates have thus definitely refuted 

 the hypothesis of Sir James Ross, who supposed that the bot'.om of the ocean from 



* Von Freeden, in Petermann's Mittheilungeu, iv. 1869. 



t Petermann's Mltthcilungen, January, 1878. 



X Despretz, " Eecherches sur le maximum de densité des dissolutions aqueuses." 



145 



