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.