1865.] 113 [Chase. 



and Cape of Good Hope, by a diminution of dip between the paral- 

 lels of 35°, and an increase in higher latitudes. 



These coincidences may well suggest the need of more extensive 

 observations in dlflFerent parts of South America, Northern Africa, 

 Southern Asia, the Pacific Ocean, and the Frigid Zones, to furnish 

 the data for determining to what extent mountain ranges, coast lines, 

 land and water radiation, winds, and ocean currents modify the theo- 

 retical phenomena of dip and declination. 



By projecting on isoclinal and isogonic charts* the magnetic cur- 

 rents as indicated by the position of the needle in different portions 

 of the globe, I have obtained the following results, which serve to 

 show the character of some of these modifications: 



I. Declination. 



1. The currents manifest a tendency to follow the lines of most 

 direct ocean communication between the warmest and the coldest 

 portions of the globe, the general declination being westerly in the 

 Atlantic, and easterly in the Pacific Ocean, 



2. The lines of no variation are apparently determined in part by 

 the land contoursf that divide the waters of the globe into two great 

 bodies. 



3. The currents are deflected by the southern pointed extremities 

 of the several continents, towards the east on the eastern shores, and 

 towards the west on the western shores of New Holland, Africa, and 

 South America. 



4. The magnetic currents, in the three respects above enumerated, 

 exhibit a lyrecise paraUellsm to the ocean tidal floio. 



5. In the comparatively narrow belt of the Atlantic Ocean, the 

 declination between the parallels of 35° reaches 30°; in the broad 

 expanse of the Pacific, the maximum within the same limits of lati- 

 tude is 15°. 



6. Between the parallels of 70° and 80° the declination becomes 



* The charts that I used, were the polyconic projection of the "Lines 

 of equal magnetic variation for the year 1858," which was constructed 

 from Evans's English Admiralty Chart, for the United States Coast 

 Survey Keport of 1859, and No. 1 and 2 of the "Admiralty Manual for 

 ascertaining and applying the deviations of the compass caused by the 

 iron in a ship." 2d edition, 1863. 



f The influence of coast lines in producing a tendency to equality of 

 declination, is beautifully shown in the United States Coast Survey Chart 

 of "Lines of equal magnetic declination on the Gulf of Mexico for the 

 epoch 1860.0." 



VOL. X. — Q 



