the Cruise of H. M.S. ' Challenger.' 465 



concretions, up to the size of an orange, or even larger, were col- 

 lected in quantity, the greater part of the red clay being usually washed 

 out. 



The surface-temperature naturally rose in passing southwards from 

 Hawaii towards the equator, and again sank from the equatorial belt 

 towards Tahiti. The isothermobaths * between 14° C. and 24° C. 

 gathered together and approached much nearer to the surface in the 

 region of the trade-winds, owing no doubt to the rapid removal of 

 the hot surface-water by evaporation and the driving action of the wind. 

 Thus the isothermobathic line of 14° C, which is at a depth of 200 

 fathoms a little to the north of Tahiti, is at a depth of 100 fathoms on 

 the line. In the Atlantic all the isothermobaths seem to participate in 

 the rise in the region of the trade-winds : it is not so in the Pacific ; 

 the lines below 14° C. uniformly sink, forming a depression which 

 extends from lat. 10° N. to lat. 10° S. ; thus the isothermobath of 5° C, 

 which may be taken as a type of these deeper lines, is found in 

 lat. 10° N". at a depth of 450 fathoms, and in lat. 10° S. at the same 

 temperature within the limits of error of observation, while in lat. 

 2° 34' N. it is found at 625 fathoms. The point where the isothermo- 

 baths gather together most markedly and approach nearest to the sur- 

 face is a little to the north of the northern border of the equatorial 

 counter current. This fall of temperature is so decided as to indicate 

 some special areas of cold water j and it may possibly be to some extent 

 due to the pressing up of deeper and therefore colder layers of the colder 

 trade-current against the hot stream. In the equatorial region between 

 lat. 10° N. and 10° S. there is a belt of water about 80 fathoms in 

 thickness at a temperature generally over 25° O. ; and the whole of 

 this water, with the exception of the narrow band of the counter 

 current, is running to the westward at the rate of from 40 to 70 miles 

 a day. 



The bottom fauna over the whole of the manganese area is very meagre, 

 both as to number of species and number of individuals. It can scarcely 

 be said, however, that its scope and extent were fairly tested ; for the 

 presence of manganese nodules is almost fatal to the working of the 

 trawl, which either becomes imbedded and impacted among them, pro- 

 ducing a destructive strain on the line, or has the bottom torn out of the 

 bag on the way up. Still in every haul in which the trawl was recovered 



* The word Isotherm having been hitherto so specially appropriated to lines passing 

 through places of equal temperature on the surface of the earth, I have found it con- 

 venient, in considering these questions of ocean temperature, to use the terms Isothermo- 

 bath and Isobathytherm ; the former to indicate a line drawn through points of equal 

 temperature in a vertical section, and the latter a line drawn through points of equal 

 depth at which a given temperature occurs. Isothermobaths are shown in a scheme 

 of a vertical section, such as Plate 16. Isobathytherms are of course projected on the 

 surface of the globe. 



2m2 



