Page:Mathematical collections and translations, in two tomes - Salusbury (1661).djvu/446

 it hath not been likewise with a perfect curiosity inquired, what its course is thorow the particular arches of the Zodiack. That therefore the Earth and Moon in running through the Zodiack, that is round the Grand Orb, do somewhat accellerate at the Moons change, and retard at its full, ought not to be doubted; for that the said difference is not manifest, which cometh to be unobserved upon two accounts; First, Because it hath not been lookt for. Secondly, Because that its possible it may not be very great. Nor is there any need that it should be great, for the producing the effect that we see in the alteration of the greatness of ebbings and flowings. For not onely those alterations, but the Tides themselves are but small matters in respect of the grandure of the subjects on which they work; albeit that to us, and to our littleness they seem great. For the addition or subduction of one degree of velocity where there are naturally 700, or 1000, can be called no great alteration, either in that which conferreth it, or in that Which receiveth it: the Water of our Mediterrane carried about by the diurnal revolution, maketh about 700 miles an hour, (which is the motion common to the Earth and to it, and therefore not perceptible to us) & that which we sensibly discern to be made in the streams or currents, is not at the rate of full one mile an hour, (I speak of the main Seas, and not of the Straights) and this is that which altereth the first, naturall, and grand motion; and this motion is very great in respect of us, and of Ships: for a Vessel that in a standing Water by the help of Oares can make v. g. three miles an hour, in that same current will row twice as far with the stream as against it: A notable difference in the motion of the Boat, though but very small in the motion of the Sea, which is altered but its seven hundredth part. The like I say of its rising, and falling one, two, or three feet; and scarcely four or five in the utmost bounds of a streight, two thousand, or more miles long, and where there are depths of hundreds of feet; this alteration is much less than if in one of the Boats that bring us fresh Water, the said Water upon the arrest of the Boat should rise at the Prow the thickness of a leaf. I conclude therefore that very small alterations in respect of the immense greatness, and extraordinary velocity of the Seas, is sufficient to make therein great mutations in relation to our smallness, and to our accidents.

.I am fully satisfied as to this particular; it remains to declare unto us how those additions and substractions derived from the diurnal Vertigo are made one while greater, and another while lesser; from which alterations you hinted that the annual period of the augmentations and diminutions of the ebbings and flowings did depend.