Page:Mathematical collections and translations, in two tomes - Salusbury (1661).djvu/421

 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 different 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 downwards; of the rest successively those that are neerest to the extreams 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 returning, and the waters that are in the extream parts gain no ground at all; save onely in case that in their rising they overflow their banks, and break forth of their first channel and receptacle; 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.

The fifth particular accident ought the more attentively to be considered, in that it is impossible to represent the effect thereof by an experiment or example; and the accident is this. In the vessels by us framed with art, and moved, as the abovenamed 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 retardment; and the self-same holds true of the acceleration, namely, that conferring on the Bark a new cause of greater 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 certain 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, reassuming a Scheme like to the former; in which if we do but suppose 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,