experienced skippers of our own country, not a few of them, in
making the voyage from the Gulf of Mexico to the islands of the Azores,
recognized that they had come as near as possible to these same islands;
although from their sea-charts they seemed to be about six hundred British
miles from them. And so, by the help of this magnetick index, it would seem
as though that geographical problem of finding the longitude, which for so
many centuries has exercised the intellects of the most learned
Mathematicians, were going to be in some way satisfied; because if the
variation for any maritime place whatever were known, the same place could
very readily be found afterward, as often as was required, from the same
variation, the latitude of the same place being not unknown._

_It seems, however, that there has been some inconvenience and hindrance
connected with the observation of this variation; because it cannot be
observed excepting when the sun or the stars are shining. Accordingly this
magnetick Mercury of the sea goes on still further to bless all
shipmasters, being much to be preferred to Neptune himself, and to all the
sea-gods and goddesses; not only does it show the direction in a dark night
and in thick weather, but it also seems to exhibit the most certain
indications of the latitude. For an iron index, suspended on its axis (like
a pair of scales), with the most delicate workmanship so as to balance in
aequilibrio, and then touched and excited by a loadstone, dips to some
fixed and definite point beneath the horizon (in our latitude in London,
for example, to about the seventy-second degree), at which it at length
comes to rest. But under the aequator itself, from that admirable agreement
and congruency which, in almost all and singular magnetical experiments,
exists between the earth itself and a terrella (that is, a globular
loadstone), it seems exceedingly likely (to say the very least), and indeed
more than probable, that the same index (again stroked with a loadstone)
will remain in aequilibrio in an horizontal position. Whence it is evident
that this also is very probable, that in an exceedingly small progress from
the South toward the North (or contrariwise) there will be at least a
sufficiently perceptible change in that declination; so that from that
declination in any place being once carefully observed along with the
latitude, the same place and the same latitude may be very easily
recognized afterward,