From thence to the elevated plateaux of the
Cordilleras, between the silver mines of micuipampa and Caxamarca, the
ancient seat of the Incas, where I observed the inclination, the line
traverses the whole of South America, which in these latitudes is as much a
magnetic 'terra incognita' as the interior of Africa.
The recent observations of Sabine* have shown that the node near the island
of St. Thomas has moved 4 degrees from east to west between 1825 and 1837.
[footnote] *See the remarkable chart of isoclinic lines in the Atlantic
Ocean for the years 1825 and 1837, in Sabine's 'Contributions to Terrestrial
Magnetism', 1840, p. 134.
It would be extremely important to know whether the opposite pole, near the
Gilbert Islands, in the South Sea, has aproached the meridian of the
Carolinas in a westerly direction. These general remarks will be sufficient
to connect the different systems of isoclinic non-parallel lines with the
great phenomenon of equilibrium which is manifested in the magnetic equator.
It is no small advantage, in the exposition of the laws of terrestrial
magnetism, that the magnetic equator (whose oscillatory change of form and
whose nodal motion exercise an influence on the inclination of the needle in
the remotest districts of the world, in consequence of the altered magnetic
latitudes)* should traverse the
p 185
ocean throughout its whole course, excepting about one fifth, and
consequently be made so much more accessible, owing to the remarkable
relations in space between the sea and land, and to the means of which we
are now possessed for determining with much exactness both the declination
and the inclination at sea.
Pages:
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387