HYDRAULIC ARCHITECTURE, BOOK III.

opposition, that this phenomenon of the flow and ebb must be attributed to the action of the sun and the moon. Furthermore, one cannot deny Mr. Newton that his sentiment is as consistent with experience as Mr. Descartes' is contrary to it; for on the day of new moons, the sea rises in all the ports of Spain, on the Ocean, and on the western coasts of Africa, as the moon, having already reached the horizon, rises above it and advances, just like the sun, toward the height of the sky. The same thing can be said regarding all the ports of the Pacific Sea on the coasts of America: it is high tide towards three o'clock on the days of the syzygies Syzygy refers to the alignment of three celestial bodies—the Earth, Moon, and Sun—in a straight line, which happens during new and full moons., that is to say, of new and full moons; whereas it should be the time of low tide if one wished to strictly adhere to the hypothesis of Mr. Descartes. We shall dispense with entering further into these purely speculative discussions, since it is much more important, for the end we propose, to insist on the matters of fact of which it is not permitted to neglect the knowledge.

The establishment of a port is the name given to the hour at which it is high tide in that port on the day of the new and full moon, from which one deduces the time of the tides for any other day.

590. To act accordingly, it should be known that the establishment of tides along a coast or in a port is the name given to the hour at which it is high tide along that coast or in that port on the day of the new and full moon. As it is very important to know this establishment to deduce the hour of other tides at any time one wishes, we shall provide at the end of this chapter a table of the establishment of tides for the principal coasts of Europe on the Ocean, with the height to which they rise there at the same time. Wishing to make use of it, one will remember that the tides occur at different hours each day, that they lag daily by 48 minutes, and that they only return to the same hours every 15 days. Since the table in question marks the tides for the days of new and full moons, one will know in advance by its means the hour at which it will be high tide at any other time, as soon as one takes into account the delay found by the rule of three The "rule of three" is a historical term for basic cross-multiplication used to find a fourth value from three known values., which we indicated in article 584.

For example, if one asks at what hour it will be high tide at Brest on September 13, 1731, one must begin by seeing in the table what the establishment of Brest is; it will be found to be 3 hours 45 minutes. One will then consult some ephemeris, or a simple almanac, to see what day of the month of