74 Temperature Sections Surveyed. 



in which the temperature slowly decreases from about 22° C. 

 at the surface, to 16° C. at a depth of 300 fathoms. This 

 stratum is represented by the protuberance or hump in Curve 

 A, Fig. 2, and Curve A, Fig. 3. Throughout this area, the 

 isotherm of 15° C. is found at an average depth of 350 fathoms, 

 that of 10° C. at 450 fathoms, that of 5° C. at about 600 fathoms, 

 and the isotherm of 2°. 5 C. at 1500 fathoms, occasionally falling 

 below 1600 fathoms. 



Section from Madeira to Tristan d' Acunha (Plate 9, 

 Table IV.). — This section traverses the Atlantic Ocean from 

 the parallel of lat. 30° N. to the parallel of lat. 37° S. along 

 the meridian of long. 20° W., and may, as regards the distribu- 

 tion of temperature, be divided into three parts — a central 

 belt, extending across the equator from lat. 15° S. to lat. 15° 

 N., bounded on each side by the belts respectively belonging 

 to the South and to the North Atlantic. 



The most remarkable feature of the central belt is the 

 rapid decrease of temperature in the surface-stratum of the 

 ocean, amounting to from 15° to 19° C. within less than 200 

 fathoms, in comparison with the much slower decrease in the 

 northern and southern belt, in the lower strata of which we 

 observe a gradual increase of temperature as we recede from the 

 equator, much more rapid, however, in the former than in the 

 latter. 



The maximum increase and decrease is seen in the diagram 

 of the section to extend from Station 346, or lat. 3° S., to Station 

 95, or lat. 13° N., limits which coincide within a few degrees with 

 the equatorial limits of the trade-winds, so that this belt may be 

 considered as identical with the equatorial belt of calms. An 

 examination of the section of the Pacific Ocean (Plate 19) shows 

 that the same phenomenon has been observed in that ocean also. 

 The increase is naturally due to the excess of solar heat in the 

 equatorial belt, the decrease to the presence of a stratum of cold 



