On the 16th we were in the latitude of 26
degrees 29 minutes south, and in the longitude of 199 degrees 32 minutes, the
variation of the needle being 8 degrees. Here we are to observe that the eastern
variation decreases, which is likewise very agreeable to Doctor Halley’s
hypothesis; which, in few words, is this: that a certain large solid body
contained within, and every way separated from the earth (as having its own
proper motion), and being included like a kernel in its shell, revolves
circularly from east to west, as the exterior earth revolves the contrary way in
the diurnal motion, whence it is easy to explain the position of the four
magnetical poles which he attributes to the earth, by allowing two to the
nucleus, and two to the exterior earth. And, as the two former perpetually alter
the situation by their circular motion, their virtue, compared with the exterior
poles, must be different at different times, and consequently the variation of
the needle will perpetually change. The doctor attributes to the nucleus an
European north pole and an American south one, on account of the variation of
variations observed near these places, as being much greater than those found
near the two other poles. And he conjectures that these poles will finish their
revolution in about seven hundred years, and after that time the same situation
of the poles obtain again as at present, and, consequently, the variations will
be the same again over all the globe; so that it requires several ages before
this theory can be thoroughly adjusted. He assigns this probable cause of the
circular revolution of the nucleus that the diurnal motion, being impressed from
without, was not so exactly communicated to the internal parts as to give them
the same precise velocity of rotation as the external, whence the nucleus, being
left behind by the exterior earth, seems to move slowly in a contrary direction,
as from east to west, with regard to the external earth, considered as at rest
in respect of the other. But to return to our voyage.
Early Australian Voyages, 1886, John Pinkerton |
