This sensible writer then states the different experiments which have been made to discover the saltness of the sea, round the shores of Britain; and proposes the following simple method of ascertaining it with tolerable certainty:—
“As it is not every person who can make himself expert in the use of common means of estimating the quantity of salt contained in sea water, I will mention a method of doing it, which is so easy and simple, that every common sailor may understand and practise it; and which, at the same time, from the trials I have made of it, seems to be as exact a method as any that has yet been thought of.—Take a clean towel, or any other piece of cloth; dry it well before the sun or before the fire, then weigh it accurately, and note down its weight; dip it in the sea water, and, when taken out, wring it a little till it will not drip when hung up to dry; weigh it in this wet state, then dry it in the sun or at the fire, and when it is perfectly dry, weigh it again: the excess of the weight of the wetted cloth above its original weight, is the weight of the sea water imbibed by the cloth; and the excess of the weight of the cloth after being dried, above its original weight, is the specific gravity of the salt retained by the cloth; and by comparing this weight with the weight of the sea water imbibed by the cloth, we obtain the proportion of salt contained in that species of sea water.”
Whoever undertakes to ascertain the quantity of salt contained in sea water, either by this or any other method, would do well to observe the state of the weather preceding the time when the sea water is taken out of the sea; for the quantity of salt contained in the water near the surface, may be influenced, both by the antecedent moisture, and the antecedent heat of the atmosphere. And this leads to the consideration of a question proposed by Aristotle,—Why are the upper parts of the sea salter and warmer than the lower? Some philosophers, admitting the fact, have followed him in attempting to explain it; whilst others have thought themselves authorized by experiment to deny the truth of the position; and those, perhaps, will argue with the greatest justness, who shall affirm that it is neither generally to be admitted, nor generally to be rejected, but that the sea in some places, and under certain circumstances, is salter and warmer at the surface, than at any considerable depth beneath it, while in many others the reverse is true. The question consists of two parts, betwixt which, though there probably is a connection, yet it is not so necessary a one as to hinder us from considering each part by itself.
With regard to the use of this salt property of sea water, it is observed, that the saltness of the sea preserves its waters pure and sweet, which otherwise would corrupt, and emit a stench like a filthy lake, and consequently that none of the myriads of creatures which now live therein could exist. From thence also the sea water becomes much heavier, and therefore ships of greater size and burden are safely borne thereon. Salt water also does not freeze so soon as fresh water, hence the seas are more free for navigation.
We shall now make a few observations on The Tides:—
Say, why should the collected main
Itself within itself contain?
Why to its caverns should it sometimes cree
And with delighted silence sleep
On the lov’d bosom of its parent deep?
Why should its num’rous waters stay
In comely discipline and fair array,
Till winds and tides exert their high commands?
Then prompt and ready to obey,
Why do the rising surges spread
Their op’ning ranks o’er earth’s submissive head,
Marching through different paths to different lands?
Prior.
The tides consist of two periodical motions of the waters of the sea, called the flux and reflux, or the flow and ebb. The cause of the tides is the attraction of the sun and moon, but chiefly of the latter; the waters of the immense ocean, forgetful, as it were, of their natural rest, move and roll in tides, obsequious to the strong attractive power of the moon, and weaker influence of the sun.
That the tides may have their full motion, the ocean in which they are produced ought to be extended from east to west 90°, or a quarter of a great circle of the earth, at least; because the places where the moon raises most, and most depresses the water, are at that distance from one another. Hence it appears, that it is only in the great oceans that such tides can be produced, and why, in the large Pacific ocean, they exceed those in the Atlantic. From this it is also obvious why the tides are not so great in the torrid zone, between Africa and America, where the ocean is narrower, as in the temperate zones on either side; and from this also, we may understand why the tides are so small in islands that are very far distant from the shore. It is manifest, that, in the Atlantic ocean, the water cannot rise on one shore, but by descending on the other; so that, on these shores, at an intermediate distance, it must continue at about a mean height between its elevation on the one, and descent on the other shore. As the tides pass over shoals, and run through streights into bays of the sea, their motion becomes more various, and their height depends on a great many circumstances. The tide that is produced in the western coast of Europe corresponds to the theory above described: thus, it is high water on the coast of Spain, Portugal, and the west of Ireland, about the third hour after the moon has passed the meridian; from thence it flows into the adjacent channels, as it finds the easiest passage. One current from it, for example, runs up by the south of England, and another comes in by the north of Scotland: they take a considerable time to move all this way, and it is high water sooner in the places to which they first come; and the tides even begin to fall at those places, while the two currents are yet going on to others that are further in their course. As they return, they are not able to raise a tide; because the water runs faster off than it returns, till by a new tide propagated from the ocean, the return of the current is stopped, and the water begins to rise again. The tide takes twelve hours to come from the ocean to London bridge, so that, when it is high water there, a new tide is already come to its height in the ocean, and, in some intermediate place, it must be low water at the same time.
In channels, therefore, and narrow seas, the progress of the tides may be, in some respects, compared to the motion of the waves of the sea. It may be observed, that when the tide runs over shoals, and flows upon flat shores, the water is raised to a greater height than in the open and deep oceans that have steep banks; because the force of its motion cannot be broken upon these level shores, till the water rises to a greater height. If a place communicates with two oceans, (or two different ways with the same ocean, one of which is a readier and easier passage than the other,) two tides may arrive at that place in different times, which, interfering with each other, may produce a greater variety of phenomena.