1480 CRUSTACEA. 



By comparing the regions of the different oceans, north and south 

 of the equator, we may arrive at the mean position of the several iso- 

 crymes, and thereby discover on a grander scale, the influence of the 

 various oceanic movements. 



For the purpose of reaching mean results, the Middle Pacific is the 

 most favourable ocean for study. This is apparent in its greater 

 extent, and the wide distance between the modifying continents; and 

 also no less in the greater actual regularity of the isocrymes. 



We hence deduce, that the mean position of the isocryme of 74 F. 

 is along the parallel of 20, this being the average between the means 

 for the North and South Pacific. In the same manner we infer that 

 the mean position of the isocryme of 68 F. is along the parallel of 27. 



The southern isocrymes of 56 and 62 F., are evidently thrown 

 into abnormal proximity by the cold waters of the south. This cur- 

 rent flows eastward over the position of the isocryme of 44 F., and 

 consequently in that latitude has nearly that temperature, although 

 colder south. Hence, it produces little effect in deflecting the line of 

 44 F. ; moreover, the line of 50 F. is not pushed upward by it. But 

 the lines of 56 and 62 F. are thrown considerably to the north by its 

 influence, and the Warm Temperate and Temperate Regions are made 

 very narrow. With these facts in view, we judge from a comparison 

 of the North and South Pacific lines, that the mean position for the 

 isocryme of 62 F. is the parallel of 32; and for 56 F., the parallel 

 of 37 ; for the isocryme of 50 F., the mean position is nearly the 

 parallel of 42; for 44 F., the parallel of 47; for 35 F., the parallel 

 of 56. There is thus a mean difference of five degrees of latitude for 

 six degrees of Fahrenheit, excepting near the equator and between 

 35 and 44 F. These results may be tabulated as follows :* 



Isocryme of 80 F., . . .. . Parallel of 6 



74, ..... "20 



" 68, ....." 27 

 " 62, . . . . . "32 



56, . . ... " 37 



" 50, ..... "42 



it 440 it 470 



^ ^ .* 



" 35, ..... "56 



* We may hence deduce the temperatures of those isocrymes to which the parallels of 

 latitude for every fire degrees would normally correspond. They would be for 20, 74 

 F. ; for 25, 70 F. ; for 30, 64-4 F. ; for 35, 58-4 F. ; for 40, 52-4 F. ; for 45, 

 46-4 F.; for 50, 41 F.; for 55, 36 F.; for 60, 31 F. 



