On all maritime coasts, except such as belong to mediterranean seas not communicating freely with the ocean, the waters are observed to be constantly changing their level. They regularly rise during about six hours, remain stationary for a few minutes, and then again descend during an equal period of time, when after having fallen to the lowest ebb, they are shortly after seen to rise again, and so on in regular and endless succession. In this manner twelve hours twenty-four minutes elapse on an average from one flood to another, so that the sea twice rises and falls in the course of a day, or rather twice during the time from one passage of the moon through the meridian to the next, a period equivalent on an average to 1-35/1000 day, or nearly twenty-five hours. Thus the tides retard from one day to another; least at new and full moon, when our more active satellite accomplishes her apparent diurnal motion round the earth in twenty-four hours, thirty-seven minutes; and most at half-moon, when, sailing more leisurely through the skies, she takes full twenty-five hours and twenty-seven minutes to perform her daily journey. As the retarding of the tides regularly corresponds with the retarding of the moon, they always return at the same hour after the lapse of fourteen days, so that at the end of each of her monthly revolutions, the moon always finds them in the same position. The knowledge of this fact is extremely useful to navigators, as it is easy to calculate the time of any tide in a port by knowing when it is high-water on the days of new and full moon. The height of the tides in the same place is as unequal and changing as the period of their intervals, and is equally dependent on the phases of the moon, increasing with her growth, and diminishing with her decrease. New and full moon always cause a higher rising of the flood (spring-tide), followed by a deeper ebb, while at half-moon the change of level is much less considerable (neap-tide). Thus in Plymouth, for instance, the neap-tides are only twelve feet high, while the ordinary spring-tides rise to more than twenty feet. The highest tides take place during the equinoxes; and eclipses of the sun and moon are also invariably accompanied by considerable floods, a circumstance which cannot fail to add to the terror of the ignorant and superstitious when a mysterious obscurity suddenly veils the great luminaries of the sky. It has also been remarked that the tides are stronger or weaker, according as the moon is at a greater or smaller distance from the earth. Thus as the height of the floods is always regulated by the relative position of the sun and moon, and the movements of these heavenly bodies can be calculated a long time beforehand, our nautical calendars are able to tell us the days when the highest spring-tides may be expected.