210 J. D. Dana on an Isothermal Oceanic Chart, 



and south of the equator, we may arrive at the mean position 

 of the several isocrymes, and thereby discover, on a grander 

 scale, the influence of the various oceanic movements. 



For the purpose of reaching mean results, the Middle Pa- 

 cific is the most favourable ocean for study. This is appa- 

 rent 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 thence 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 current flows eastward over the position of the 

 isocryme of 44° F., and consequently in that latitude has 

 nearly this temperature, although colder to the south. Hence 

 it produces little effect in deflecting the line of 44° F. ; more- 

 over 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 pa- 

 rallel of 32°; and for 56° F., the parallel of 37° F. ; 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 la- 

 titude for six degrees of Fahrenheit, excepting near the equa- 

 tor, and between 35 and 44° F. These results may be ta- 

 bulated as follows :* — 



* We may hence deduce the temperature of those isocrymes to which the 

 parallels of latitude for every five degrees would normally correspond. They 

 would be for 20°, 71' F. ; for 25°, 70° P. ; for 30°, 64-4° P. ; for 35°, 581' P. ; 

 for tO , r,2-4° F. ; for 45°, 4G4° P. ; for 50°, 41° F. ; for 55°, 36° P. ; for 60°, 

 3] F. 



