1468 CRUSTACEA. 



INDIAN OCEAN. Isocrymes of 74 and 68 F. Off the south extre- 

 mity of Madagascar, in 27 33' south, 47 17' east, on August 4th, 

 Vaillant found the temperature 69-26 F. ; and in 29 34' south, 

 46 46' east, the temperature of 67-84 F.; off South Africa, August 

 12, in 34 42' south, 27 25' east, the temperature 63-5 F.; on Au- 

 gust 14, in 35 41' south, 22 34' east, a temperature of 63-3 F.; 

 while off Cape Town, two hundred miles to the west, the temperature 

 was 50 to 54 F. 



In the above review, we have mentioned only a few of the obser- 

 vations which have been used in laying down the lines, having 

 selected those which bear directly on some positions of special interest, 

 as regards geographical distribution. 



The Chart also contains the heat-equator, a line drawn through 

 the positions of greatest heat over the oceans. It is a shifting line, 

 varying with the seasons, and hence, there is some difficulty in fixing 

 upon a course for it. We have followed mainly the Chart of Berg- 

 haus. But we have found it necessary to give it a much more 

 northern latitude in the western Pacific, and also a flexure in the 

 western Atlantic, both due to the currents from the south that flow 

 up the southern continents. 



Vaillant, passing from Guayaquil to the Sandwich Islands, found 

 the temperature, after passing the equator, .slowly increase from 76 

 F., August 19, in 2 39' north, 91 58' west (of Greenwich), to 81-9 

 F., in August 31, 11 15' north, 107 3' west, after which it was not 

 above 80 F. The same place in the ocean which gave Vaillant 76 

 F., in August, afforded Fitzroy (4 north, 96 west), on March 26 

 (when the sun had long been far north), 82 i F. This shows the 

 variations of temperature that take place with the change of season. 



REMAKES ON THE SEVERAL REGIONS. 



The form and varying breadth of the different regions, and the 

 relations between the sea-temperatures of coasts in different latitudes, 

 which they exhibit, are points demanding special remark. 



1. Atlantic Torrid Region, between 74 F. north, and 74 F. south. 

 The form of this region is triangular, with the vertex of the triangle 



