The _third_, and most remarkable motion of the sea, is the tide; which is a regular swell of the ocean every 12 hours, accounted for from the principal of gravitation. The sagacious Kepler long ago conjectured, that the earth and moon, and every particle of them, mutually gravitate towards each other, and are the cause of the tides. If, says he, the earth ceased to attract its waters towards itself, all the water in the ocean would rise and flow into the moon: the sphere of the moon’s attraction extends to our earth, and draws up the water. This, at that time, was mere conjecture; for Sir Isaac Newton was the first who clearly pointed out the cause of this phenomenon. On the shores of the ocean, and in bays, creeks, and harbors, which communicate freely with it, the waters rise above their mean height twice a day, and as often sink below it, forming what is called a _flood_ and an _ebb_, a _high_ and _low water_. It has been stated, that in the middle of the sea the tide seldom rises higher than one or two feet; but, on the coast, it frequently reaches to the height of 45 feet, and, in some places, even to more. At Plymouth, it is sometimes 21 feet between the greatest and least depth of the water in the same day, and sometimes only 12 feet. When the sun and moon act conjointly on the tides, which is at the change and full of the moon, they are stronger and run higher than at other times, and are called _spring tides_; but when the sun and moon are 90 degrees apart, their attractive powers, being in opposition to each other, occasion the tides to be weaker and lower than at other times, and these are called _neap tides_. The word _neap_ is derived from the Saxon; it signifies low, decrescent, and is used only of the tide. These different heights of tide are observed to succeed each other in a regular series, diminishing from the greatest to the least, and then increasing from the least to the greatest, according to the age and situation of the moon. “The moon turns ocean in his bed, From side to side, in constant ebb and flow, And purifies from stench his watery realms.” Sir Isaac Newton calculated the attractive powers of the sun and moon on the tides, and found the attraction of the latter to be about three times greater than that of the former. Water is found to exist in four states: namely, solid, or ice; liquid, or water; vapor, or steam; and in a state of composition in other bodies. The younger Lemery observes, that ice is only the re-establishment of the parts of water in their natural state; that the mere absence of fire is sufficient to account for this re-establishment; and that the fluidity of water is a real fusion, like metals exposed to the fire; differing only in this, that a greater quantity of fire is necessary to the one than the other.