198 J. D. Dana on an Isothermal Oceanic Chart, 



the north, owing to the expansion of the Gulf Stream in that 

 direction, and forms a large fold between Britain and Ice- 

 land ; it then bends south again and curves around to the 

 west coast of Ireland. 



Isocryme of 35° F. — This line has a bend between Nor- 

 way and Iceland like that of 44°, and from the same cause, 

 — the influence of the Gulf Stream. But its exact position 

 in this part has not been ascertained. 



2. South Atlantic. — Isocryme of 74° F. — This line be- 

 gins just south of Bahia, where Fitzroy found in August 

 (the last winter month) a temperature of 74° to 75^° F. 

 During the same month he had 75|° to 76^° F. at Pernam- 

 buco, live degrees to the north. Off Bahia, the temperature 

 was two degrees warmer than near the coast, owing to the 

 warm tropical current, which bends the isocryme south to 

 latitude 17° and 18° and the cold waters that come up the 

 coast from the south. The line gradually rises northward, 

 as it goes west, and passes the equator on the meridian of 

 Greenwich. Sabine, in a route nearly straight from Ascen- 

 sion Island, in 8° south, to the African coast under the equa- 

 tor, obtained in June (not the coldest winter month) the 

 temperatures 78°, 77°, 74°, 72-8°, 72-5°, 73°, the tempera- 

 ture thus diminishing on approaching the coast, although 

 at the same time nearing the equator, and finally reaching 

 it within a few miles. These observations in June shew 

 that the isocryme of 74° F. passes north of the equator. The 

 temperatures mentioned in Maury's chart afford the same 

 conclusion, and lead to its position as laid down. 



Isocryme of 68° F.— On October 23 to 25, 1834, Mr 

 D. J. Browne, on board the U.S. ship Erie, found the tem- 

 perature of the sea, on entering the harbour of Rio Janeiro, 

 67 f to 68£ F. Fitzroy, on July 6, left the harbour with the 

 sea-temperature 70 \° F. Beechy, in August 1825, obtained 

 the temperature 68-16° to 69-66 ' F. off the harbour. The 

 isocryme of 68" F. commences therefore near Bio, not far 

 south of this harbour. Eastward of the harbour, the tempera- 

 ture increases two to four degrees. In July, Fitzroy carried 

 a temperature above 68 as far south as 33° 16' south, longi- 

 tude 50 10' west, the water giving at this time 68£° to 69}/ 



