Salusbury, Thomas Mathematical collections and translations 1667

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392

tions; the one is its rising and falling alternately towards the
one and other extremity; the other is its moving and running, to
so speak, Horizontally forwards and backwards. Which two dif­
ferent motions differently reside in divers parts of the Water:
for its extream parts are those which most eminently rise and fall;
those in the middle never absolutely moving upwards and down­
wards, of the rest successively those that are neerest to the ex­
treams rise and fall proportionally more than the remote: but on
the contrary, touching the other progressive motion forwards
and backwards, the middle parts move notably, going and re­
turning, and the waters that are in the extream parts gain no
ground at all; save onely in case that in their rising they over­
flow their banks, and break forth of their first channel and re­
ceptacle; but where there is the obstacle of banks to keep them
in, they onely rise and fall; which yet hindereth not the waters
in the middle from fluctuating to and again; which likewise
the other parts do in proportion, undulating more or lesse,
according as they are neerer or more remote from the middle.

Water riseth &
falleth in the ex­
tream parts of the
Vessel, and runneth
to and fro in the
midst.

An accident of
the Earths motions
impossible to be re­
duced to practice
by art.

The fifth particular accident ought the more attentively to be
considered, in that it is impossible to represent the effect there­
of by an experiment or example; and the accident is this. In
the vessels by us framed with art, and moved, as the above­
named Bark, one while more, and another while lesse swiftly,
the acceleration and retardation is imparted in the same manner
to all the vessel, and to every part of it; so that whilst v. g. the
Bark forbeareth to move, the parts precedent retard no more
than the subsequent, but all equally partake of the same re­
tardment; and the self-same holds true of the acceleration,
namely, that conferring on the Bark a new cause of grea­
ter velocity, the Prow and Poop both accelerate in one and
the same manner. But in huge great vessels, such as are the very
long bottomes of Seas, albeit they also are no other than cer­
tain cavities made in the solidity of the Terrestrial Globe,
it alwayes admirably happeneth, that their extreams do not
unitedly equall, and at the same moments of time increase
and diminish their motion, but it happeneth that when one of its
extreames hath, by vertue of the commixtion of the two
Motions, Diurnal, and Annual, greatly retarded its velocity,
the other extream is animated with an extream swift motion.
Which for the better understanding of it we will explain, reas­
suming a Scheme like to the former; in which if we do but sup­
pose a tract of Sea to be long, v. g. a fourth part, as is the arch
B C [in Fig. 2.] because the parts B are, as hath been already
declared, very swift in motion, by reason of the union of the
two motions diurnal and annual, towards one and the same way,