Archimedes Texts

397
of which is (as hath often been said) the determinate accelera
tion 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 sour hours. The other is that which dependeth on the pro
per gravity of the Water, which being once commoved by the
primary cause, seeketh, in the next place, to reduce it self to Æ
quilibrium, with iterated reciprocations; which are not deter
mined by one sole and prefixed time; but have as many varie
ties 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 secon
dary, 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
and 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 altoge
ther 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 dis
imbogue 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 moti
ons, dependent not onely on the differenr positions and longi
tudes, but very much also upon the different profundities of the
Seas, which have the said intercourse there do happen at some
times 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