1872.] 



' Shearwater' Scientific Researches. 



613 



under the influence of the South-African current. The February Isotherm 

 of 70° and the August Isotherm of G0° follow a nearly similar course about 

 5° further South ; as do the February Isotherm of 65°, and the August Iso- 

 therm of 55° near the parallel of 40° S. The marked regularity in the 

 distribution of these lines and in their seasonal movements, as compared 

 with the strong deflection of the Isotherms to the north of 50° S., seems to 

 indicate that the Thermal influence of the Brazil current dies out sooner 

 than its moving force ; and this, it seems probable, may be attributed to 

 the circumstance that its superheated stratum has only the ordinary thick- 

 ness of that of Oceanic water, whilst in the Gulf-stream the excess of heat 

 extends to a much greater depth (§ 12/). For it is obvious that the 

 thinner the superheated stratum, the more speedily will its temperature 

 assimilate itself to that of the overlying Atmosphere ; and that such is the 

 case with the Brazil current, appears from the comparatively small south- 

 ward deflection of the August (winter) Isotherms of 75° and 70°, while 

 the Southern Isotherm of 80° moves northwards in that month, so as in 

 September to be found wholly to the north of the Equator, continuing so 

 in great part through October and November. 



124. In our examination of the course of the Isotherms of the North 

 Atlantic, it will be convenient to restrict ourselves, in the first instance, to 

 that portion of it which, with the large contribution supplied by the Equa- 

 torial Current from the southern side of the Equator, becomes the feeder of 

 the Gulf-stream. — In February and March, the northern Isotherm of 80° 

 follows pretty closely the northern boundary of the Equatorial Current ; 

 but it does not extend into the Caribbean Sea, the general temperature of 

 which during those months does not seem to be above 76°, or about 

 the average of that of the open ocean between the same parallels. In the 

 Gulf of Mexico the surface-temperature falls still lower, especially within 

 the area over which the comparatively cold water brought down by the 

 Mississippi diffuses itself ; the temperature of the head of the Gulf being 

 thus reduced even below 70°. — Now since it is at this period of the year 

 that the temperature of the Southern portion of the Equatorial Current 

 is at its highest, averaging about 81|°, it is obvious that the surplus 

 heat which it carries towards the Caribbean Sea and the Gulf of Mexico 

 is for the most part dissipated before its water re-issues into the Atlantic 

 as the Gulf-stream. This will be seen to be a fact of some importance 

 with reference to the Thermal effect of the Atmosphere upon the surface- 

 temperature of the Sea. — The February and March Isotherm of 70° 

 between the meridians of 35° and 75° W. keeps very close to the parallel of 

 28° N., passing directly across the area of the Sargasso Sea ; east of 

 Long. 35°, however, it bends southward, under the influence of the 

 colder North-African current ; whilst to the west of Long. 75° it turns 

 abruptly to the North, under the influence of the Gulf-stream, which 

 issues from the Florida Channel during these months at a temperature 

 of about 75°. Impinging, as it were, on the Isotherm of 70°, the 



