CHAPTER I. ON THE FLOW AND EBB OF THE SEA.

Part II. Volume II. B

of September the full moon will occur, and it will be found to be on the 5th, without worrying about the exact hour of this phase; for since an entire day of the moon's movement produces only a 48-minute delay in the tides, it is enough to know the approximate time of the full or new moon to avoid any significant error in the intended calculation. It remains, then, to know by how much the tide will be delayed over 8 days—that is to say, from the 5th until the 13th. But we have found (article 584) that this delay for 8 days is six hours and 24 minutes; consequently, it will be high tide at Brest on September 13, 1751, not at 3:45 The "establishment" or standard high tide time for Brest on the day of a new or full moon., but at 10:09. As for the low tide of the same day, as it must occur a little more than 6 hours earlier or later, depending on whether one wants the one preceding or following, it will occur around 4:00, both in the morning and the evening.

By this calculation, one can take measures to be in a state to work on that day on some project that could only be executed at low tide; two or three days later, one will have even more convenience, since the two tides will occur around six o'clock in the morning and evening, and consequently both during the daylight.

The high tides occur two days after the time of the syzygies Syzygy: the alignment of the Earth, Moon, and Sun during new and full moons., and the weak tides during the quadratures Quadrature: when the Moon is at a right angle to the Sun relative to Earth, occurring during the first and last quarter phases.: their difference is about one-third.

PLATE III. FIGURE Y.

591. Another circumstance of the flow and ebb that particularly interests those charged with the direction of works carried out in seaports is the amount by which the tides rise and fall: a circumstance that depends greatly on the layout of the coastal areas, and too visibly on the movements of the sun and the moon not to be convinced that they are the sole causes of the flow and ebb during the time of the syzygies, or the new and full moons. When these two celestial bodies A and B are in conjunction and act in concert, their combined action, having much more force at that time, also occasions much greater tides; this is why they are distinguished from others by being called fortes eaux Literally "strong waters.", vives eaux Literally "living waters"; the standard term for spring tides., malines or reverdies Regional nautical terms for spring tides., which occur twice each month. It should be observed that these spring tides do not occur precisely on the days of the new and full moons, but two days later, because the sea acquires an acceleration of movement during that time which causes it to rise higher than usual, even though the moon is found to be a little removed from the sun or from its directly opposite point; thereafter, the tides decrease from day to day