The water of the sea flows for about six hours from south to north; during which period it gradually swells, so that it enters the mouths of rivers, and counteracts the natural current from their sources. It then remains stationary for about a quarter of an hour; after which it ebbs for six hours ; a similar pause of 15 minutes takes place, when it flows, and ebbs alternately. Thus, the tide rises and falls once in the space of 12 hours and 48 minutes, which period constitutes a lunar day ; as the moon passes the meridian of the earth about 48 minutes later, each succeeding day. The motion of the tide farther influenced by the moon : hence, if that luminary be in the first and third quarters, or when it is new and full, they are high and swift, being then called springtides : on the contrary, when the moon is in the second and last quarters, they neither rise so high, nor flow with such rapidity; and are therefore termed neap-tides.
Various theories have been formed, to account for the phenomena of tides ; the ancients attributing them to the sun; while some modern inquirers ascribe them to the liquefaction of the ice and snow, in the polar regions; but they are now generally understood to depend on 'he principle of gravitation.
In the 1st vol. of the Repertory of Arts, etc. we meet with an account of a Tide-wheel, that may be adapted to any kind of mill ; and which was invented by Mr. Robert Leslie. As a mere description of its constituent parts would convey but an inadequate idea of its mechanism, the curious reader will resort to the work above cited; where the whole contrivance is illustrated with an engraving. We shall, therefore, only remark, that such wheel, if intended for mills, is by the inventor stated to be preferable to all others turned by the tide ; for its velocity is not only greater and more uniform, but a similar quantity of water also constantly acts on the wheel, which is so constructed, as to turn the same way, both with the ebb and flow, and in a more simple manner than any hitherto contrived. Thus, it moves hori-zontally, while the generality of tide-wheels revolve vertically; and the perpendicular shaft may be carried to such a height, as to admit the spur-wheel, or any other piece of mechanism, to be wholly above the surface of the water. - Lastly, it is asserted to be less expensive than other wheels of a similar construction, and may be advantageously employed for raising-water ; its velocity being fully sufficient, without any cog-wheels, or wallowers.