Page:Mathematical collections and translations, in two tomes - Salusbury (1661).djvu/426

 of which is (as hath often been said) the determinate acceleration and retardation of the parts of the Earth, from whence the Waters have a determinate period put to their decursions towards the East, and return towards the West, in the time of twenty four hours. The other is that which dependeth on the proper gravity of the Water, which being once commoved by the primary cause, seeketh, in the next place, to reduce it self to Æquilibrium, with iterated reciprocations; which are not determined by one sole and prefixed time; but have as many varieties of times as are the different lengths and profundities of the receptacles, and Straights of Seas; and by what dependeth on this second principle, they would ebbe and flow, some in one hour, others in two, in four, in six, in eight, in ten, &c. Now if we begin to put together the first cause, which hath its set Period from twelve hours to twelve hours, with some one of the secondary, that hath its Period ''verb. grat''. from five hours to five hours, it would come to passe, that at sometimes the primary cause and secondary would accord to make impulses both one the same way; and in this concurrency, and (as one may call it) unanimous conspiration the flowings shall be great. At other times it happening that the primary impulse doth, in a certain manner, oppose that which the secondary Period would make, and in this contest one of the Principles being taken away, that which the other would give, will weaken the commotion of the Waters, and the Sea will return to a very tranquil State, and almost immoveable. And at other times, according as the two aforesaid Principles shall neither altogether contest, nor altogether concur, there shall be other kinds of alterations made in the increase and diminution of the ebbing and flowing. It may likewise fall out that two Seas, considerably great and which communicate by some narrow Channel, may chance to have, by reason of the mixtion of the two Principles of motion, one cause to flow at the time that the other hath cause to move a contrary way; in which case in the Channel, whereby they disimbogue themselves into each other, there do extraordinary conturbations insue, with opposite and vortick motions, and most dangerous boilings and breakings, as frequent relations and experiences do assure us. From such like discordant motions, dependent not onely on the different positions and longitudes, but very much also upon the different profundities of the Seas, which have the said intercourse there do happen at sometimes different commotions in the Waters, irregular, and that can be reduced to no rules of observation, the reasons of which have much troubled, and alwayes do trouble Mariners, for that they meet with them without seeing either impulse of winds, or