XII. And it is very worthy of remarking, that even after experience
had made those first discoveries, on which the theory and practice of
arts are founded, those primitive lights were in general found to be
insufficient to enable the understanding to make further advances in
them, but it was rather found to be necessary, that the same experience
should continue as they proceeded on, to direct their steps, and correct
their errors. I will explain what I mean, by two examples taken from
navigation.
XIII. The first regards the direction, or pointing of the magnetical
needle to the pole. This admirable property, which was totally unknown to
the antients, was discovered in the thirteenth century, and immediately
applied to the improvement of navigation. Upon its first discovery,
the philosophers, according to their wonted custom of pretending to
discern the causes of things, imputed this effect, as derived from an
occult sympathy with the pole, contained in the very essence, form, and
substance of the loadstone; and as this is supposed to be invariable,
they concluded, that the direction must infallibly be invariable also.
They continued in this good faith for about four hundred years, at the
end of which long period, Crinon, a pilot of Dieppe as some say, or
Cabot, a Venetian Navigator as others believe, was the first who observed
the declinations of the magnetical needle from the true north, that
is, that it did not always point directly to the pole, but declined in
different places, sometimes towards the west, and at others towards the
east. The philosophers heard this novelty with great disgust, because it
gave the lye to some of their most established maxims, and therefore they
set about to contradict it with all their force. But in the end, they
were obliged to submit to repeated experiments, authorized and confirmed,
by the testimonies of people of undoubted credit.
XIV. Having afterwards discovered, that under the meridian of the Azores
or western islands, there was no variation of the magnetical needle
at all, the astronomers and geographers, thought they had found out a
fixed station, whereat to commence the first meridian, which had before
been counted arbitrarily, from whatever place they chose to begin
it. But this idea soon vanished, for a little while afterwards, they
discovered two other meridians, where there was no variation; the one
at a head-land near _the Cape of Good Hope_, to which, on this account
they gave the name of _Needle Cape_; and another, at the spot where the
city of Canton is situated. Upon this, they thought they had found out a
certain principle, whereon to ground a compleat system for calculating or
computing variations, by graduating them for the intermediate stations,
in proportion to their greater or less distance from the mean space
between the two places where there was no variation.
XV. But as nature frequently mocks the ideal propositions that are
fabricated by the brain of man, this time of rejoicing lasted but a few
years, for they discovered, that this declination of the magnetic needle,
varied more or less at the same place at different times, and that this
change of variation was perpetual. This discovery, not only demolished
the antecedent imagined rules, but nearly took away all hope, of their
ever in future, finding out any certain one for their guidance; and
this, notwithstanding many men, eminent for their skill in physics and
mathematics, have long, and still do, labour at accomplishing it.
XVI. In this instance, may be seen the fallibility of the most plausible
reasonings unaccompanied by experiments. And we shall see the same, in
the other we are about to mention, which relates to the flux and reflux
of the sea.
XVII. As the flux and reflux of the sea, is evident and apparent to
all those who inhabit near the sea-coasts, and the course and changes
of the Moon to all mankind, it was easy and natural, to observe the
correspondence there was between the movements of the one and the other;
that is, that the rising and falling of the tide, keeps pace with the
rising and setting of the moon; and it is probable, that the first people
who remarked this, fancied that by this single observation, they had
discovered the system of these admirable movements. But this delusion
was but short-lived, for they soon afterwards observed, that within the
space of the same lunar month, there was a great disproportion between
the rising and falling of the tide, and that it flowed higher, and ebbed
lower, at the new and full moon, than it did at the quarters. But when
they had advanced this step, it is likely they gave themselves credit
for having discovered the whole secret, and supposed, that they knew all
that was to be known of the matter; and when they observed there was so
exact a correspondence, between the motions of the tide, and the phases
of the moon, they did not entertain the least doubt, but that they were
totally influenced by this planet. But experience also undeceived mankind
with respect to this error, for they found out, that there was another
variation of the tides, which did not depend upon the revolutions of the
moon, but upon those of the sun; that is, that supposing other things to
be equal, they rise higher, at, or near the equinoxes, than they do at
the solstices. This gave them to understand, that the moon did not reign
so despotic over the tides, as to exclude the sun from all share in the
dominion of them.
XVIII. But after all this was ascertained, they found, that by
trusting the bringing of ships into barr’d or tide harbours, to the
mere combination of the before-nam’d observations, they fell into very
dangerous errors; for that there are two other variations, which are very
considerable, and especially one of them, and which can’t be comprehended
under any general rule whatsoever; the one is with respect to the time of
full sea; and the other, regards the quantum of rise of the water. The
time of full sea varies at different ports, even under the same meridian,
and does not happen in all of them at the same hour. They vary also with
respect to the magnitude of them, because there is an enormous difference
in the height they flow at some ports, to what they do at others, for
the water will rise more than ten fathoms upon some coasts, and not above
a few feet on others, and in others again, the rise is hardly perceptible.