COMPASS.
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.