Every inhabitant of a maritime country like Great Britain is more or less familiar with the phenomena now under our consideration, but beyond knowing the general fact, that the Moon has something to do with the tides, their physical history is not so well understood as it ought to be.

The phenomena of the tides are very frequently attributed to the attraction of the Moon, whereby the waters of the ocean are drawn towards that side of the Earth on which our satellite happens to be situated; in fact, that it is high water when the Moon is on or near the meridian of the place of observation.

This, though to a certain extent true, by no means adequately represents the facts of the case, for high water is not only produced on that side of the Earth immediately under the Moon, but also on the opposite side at the same time. The two tides are therefore separated from each other by 180°, or by a space equal to half the circumference of the globe. The diurnal rotation of the Earth, causing every portion of its surface to pass successively under the tidal waves, in about 24h., it follows that there are everywhere 2 tides daily, with an interval of about 12 hours between each; whereas, if the common supposition were correct, there would be only.