WAVE
, in Physics, a volume of water elevated by the action of the wind &c, upon its surface, into a state of fluctuation, and accompanied by a cavity. The extent from the bottom or lowest point of one cavity, and across the elevation, to the bottom of the next cavity, is the breadth of the Wave.
Waves are considered as of two kinds, which may be distinguished from one another by the names of natural and accidental Waves. The natural Waves are those which are regularly proportioned in size to the strength of the wind which produces them. The accidental Waves are those occasioned by the wind's reacting upon itself by repercussion from hills or high shores, and by the dashing of the Waves themselves, otherwise of the natural kind, against rocks and shoals; by which means these Waves acquire an elevation much above what they can have in their natural state.
Mr. Boyle proved, by numerous experiments, that the most violent wind never penetrates deeper than 6 feet into the water; and it seems a natural consequence of this, that the water moved by it can only be elevated to the same height of 6 feet from the level of the surface in a calm; and these 6 feet of elevation being added to the 6 of excavation, in the part from whence that water so elevated was raised, should give 12 feet for the utmost elevation of a Wave. This is a calculation that does great honour to its author; as many experiments and | observations have proved that it is very nearly true in deep seas, where the Waves are purely natural, and have no accidental causes to render them larger than their just proportion.
It is not to be understood however, that no Wave of the sea can rise more than 6 feet above its natural level in open and deep water; for Waves vastly higher than these are formed in violent tempests in the great seas. These however are not to be accounted Waves in their natural state, but as compound Waves formed by the union of many others; for in these wide plains of water, when one Wave is raised by the wind, and would elevate itself up to the exact height of 6 feet, and no more, the motion of the water is so great, and the succession of Waves so quick, that while this is rising, it receives into it several other Waves, each of which would have been at the same height with itself; these run into the first Wave one after another, as it is rising; by which means its rise is continued much longer than it naturally would have been, and it becomes accumulated to an enormous size. A number of these complicated Waves rising together, and being continued in a long succession by the continuation of the storm, make the Waves so dangerous to ships, which the sailors in their phrase call mountains high.
Different Waves do not disturb one another when they move in different directions. The reason is, that whatever figure the surface of the water has acquired by the motion of the Waves, there may in that be an elevation and depression; as also such a motion as is required in the motion of a Wave.
Waves are often produced by the motion of a tremulous body, which also expand themselves circularly, though the body goes and returns in a right line; for the water which is raised by the agitation, descending, forms a cavity, which is every where surrounded with a rising.
The Motion of the Waves, makes an article in the Newtonian philosophy; that author having explained their motions, and calculated their velocity from mathematical principles, similar to the motion of a pendulum, and to the reciprocation of water in the two legs of a bent and inverted syphon or tube.
His proposition concerning such canal or tube is the 44th of the 2d book of his Principia, and is this: “If water ascend and descend alternately in the erected legs of a canal or pipe; and a pendulum be constructed, whose length between the point of suspension and the centre of oscillation, is equal to half the length of the water in the canal; then the water will ascend and descend in the same times in which the pendulum oscillates.” The author hence infers, in prop. 45, that the velocity of Waves is in the subduplicate ratio of their breadths; and in prop. 46, he proceeds “To find the velocity of Waves,” as follows: “Let a pendulum be constructed, whose length between the point of suspension and the centre of oscillation is equal to the breadth of the Waves; and in the time that the pendulum will perform one single oscillation, the Waves will advance forward nearly a space equal to their breadth. That which I call the breadth of the Waves, is the transverse measure lying between the deepest part of the hollows, or between the tops of the ridges. Let ABCDEF represent the surface of stagnant water ascending and descending in successive Waves; also let A, C, E, &c, be the tops of the Waves; and B, D, F, &c, the intermediate hollows. Because the motion of the Waves is carried on by the successive ascent and descent of the water, so that the parts of it, as A, C, E, &c, which are highest at one time, become lowest immediately after; and because the motive force, by which the highest parts descend and the lowest ascend, is the weight of the elevated water, that alternate ascent and descent will be analogous to the reciprocal motion of the water in the canal, and observe the same laws as to the times of its ascent and descent; and therefore (by prob. 44, above mentioned) if the distances between the highest places of the Waves A, C, E, and the lowest B, D, F, be equal to twice the length of any pendulum, the highest parts A, C, E, will become the lowest in the time of one oscillation, and in the time of another oscillation will ascend again. Therefore between the passage of each Wave, the time of two oscillations will intervene; that is, the Wave will describe its breadth in the time that the pendulum will oscillate twice; but a pendulum of 4 times that length, and which therefore is equal to the breadth of the Waves, will just oscillate once in that time. Q. E. I.
“Corol. 1. Therefore Waves, whose breadth is equal to 39 1/8 inches, or 3 25/96 feet, will advance through a space equal to their breadth in one second of time; and therefore in one minute they will go over a space of 195 5/8 feet; and in an hour a space of 11737 feet, nearly, or 2 miles and almost a quarter.
“Corol. 2. And the velocity of greater or less Waves, will be augmented or diminished in the subduplicate ratio of their breadth.
“These things (Newton adds) are true upon the supposition, that the parts of water ascend or descend in a right line; but in fact, that ascent and descent is rather performed in a circle; and therefore I propose the time defined by this proposition as only near the truth.”
Stilling Waves by means of Oil. This wonderful property, though well known to the Ancients, as appears from the writings of Pliny, was for many ages either quite unnoticed, or treated as fabulous by succeeding philosophers. Of late it has, by means of Dr. Franklin, again attracted the attention of the learned; though it appears, from some anecdotes, that seafaring people have always been acquainted with it. In Martin's description of the Western Islands of Scotland, we have the following passage: “The steward of Kilda, who lives in Pabbay, is accustomed, in time of a storm, to tie a bundle of puddings, made of the fat of seafowl, to the end of his cable, and lets it fall into the sea behind his rudder. This, he says, hinders the Waves from breaking, and calms the sea.” Mr. Pennant, in his British Zoology, vol. iv, under the article | Seal, takes notice, that when these animals are devouring a very oily fish, which they always do under water, the Waves above are remarkably smooth; and by this mark the fishermen know where to find them. Sir Gilbert Lawson, who served long in the army at Gibraltar, assured Dr. Franklin, that the fishermen in that place are accustomed to pour a little oil on the sea, in order to still its motion, that they may be enabled to see the oysters lying at its bottom, which are there very large, and which they take up with a proper instrument. A similar practice obtains among fishermen in various other parts, and Dr. Franklin was informed by an old sea-captain, that the fishermen of Lisbon, when about to return into the river, if they saw too great a surf upon the bar, would empty a bottle or two of oil into the sea, which would suppress the breakers, and allow them to pass freely.
The Doctor having revolved in his mind all these pieces of information, became impatient to try the experiment himself. At last having an opportunity of observing a large pond very rough with the wind, he dropped a small quantity of oil upon it. But having at first applied it on the lee side, the oil was driven back again upon the shore. He then went to the windward side, and poured on about a tea-spoon full of oil; this produced an instant calm over a space several yards square, which spread amazingly, and extended itself gradually till it came to the lee-side; making all that quarter of the pond, perhaps half an acre, as smooth as glass. This experiment was often repeated in different places, and always with success. Our author accounts for it in the following manner:
“There seems to be no natural repulsion between water and air, to keep them from coming into contact with each other. Hence we find a quantity of air in water; and if we extract it by means of the air pump, the same water again exposed to the air will soon imbibe an equal quantity.—Therefore air in motion, which is wind, in passing over the smooth surface of water, may rub as it were upon that surface, and raise it into wrinkles; which, if the wind continues, are the elements of future Waves. The smallest Wave once raised does not immediately subside and leave the neighbouring water quiet; but in subsiding raises nearly as much of the water next to it, the friction of the parts making little difference. Thus a stone dropped into a pool raises first a single Wave round itself, and leaves it, by sinking to the bottom; but that first Wave subsiding raises a second, the second a third, and so on in circles to a great extent.
“A small power continually operating, will produce a great action. A finger applied to a weighty suspended bell, can at first move it but little; if repeatedly applied, though with no greater strength, the motion increases till the bell swings to its utmost height, and with a force that cannot be resisted by the whole strength of the arm and body. Thus the small first raised Waves being continually acted upon by the wind, are, though the wind does not increase in strength, continually increased in magnitude, rising higher and extending their bases, so as to include a vast mass of water in each Wave, which in its motion acts with great violence. But if there be a mutual repulsion between the particles of oil, and no attraction between oil and water, oil dropped on water will not be held together by adhesion to the spot whereon it falls; it will not be imbibed by the water; it will be at liberty to expand itself; and it will spread on a surface that, besides being smooth to the most perfect degree of polish, prevents, perhaps by repelling the oil, all immediate contact, keeping it at a minute distance from itself; and the expansion will continue, till the mutual repulsion between the particles of the oil is weakened and reduced to nothing by their distance.
“Now I imagine that the wind blowing over water thus covered with a film of oil cannot easily catch upon it, so as to raise the first wrinkles, but slides over it, and leaves it smooth as it finds it. It moves the oil a little indeed, which being between it and the water, serves it to slide with, and prevents friction, as oil does between those parts of a machine that would otherwise rub hard together. Hence the oil dropped on the windward side of a pond proceeds gradually to leeward, as may be seen by the smoothness it carries with it quite to the opposite side. For the wind being thus prevented from raising the first wrinkles that I call the elements of Waves, cannot produce Waves, which are to be made by continually acting upon and enlarging those elements; and thus the whole pond is calmed.
“Totally therefore we might suppress the Waves in any required place, if we could come at the windward place where they take their rise. This in the ocean can seldom if ever be done. But perhaps something may be done on particular occasions to moderate the violence of the Waves when we are in the midst of them, and prevent their breaking when that would be inconvenient. For when the wind blows fresh, there are continually rising on the back of every great Wave a number of small ones, which roughen its surface, and give the wind hold, as it were, to push it with greater force. This hold is diminished by preventing the generation of those small ones. And possibly too, when a Wave's surface is oiled, the wind, in passing over it, may rather in some degree press it down, and contribute to prevent its rising again, instead of promoting it.
“This, as mere conjecture, would have little weight, if the apparent effects of pouring oil into the midst of Waves were not considerable, and as yet not otherwise accounted for.
“When the wind blows so fresh, as that the Waves are not sufficiently quick in obeying its impulse, their tops being thinner and lighter, are pushed forward, broken, and turned over in a white foam. Common Waves lift a vessel without entering it; but these, when large, sometimes break above and pour over it, doing great damage.
“That this effect might in any degree be prevented, or the height and violence of Waves in the sea moderated, we had no certain account; Pliny's authority for the practice of seamen in his time being slighted. But discoursing lately on this subject with his excellency Count Bentinck of Holland, his son the honourable Captain Bentinck, and the learned professor Allemand (to all whom I showed the experiment of smoothing in a windy day the large piece of water at the head of the | green park), a letter was mentioned which had been received by the Count from Batavia, relative to the saving of a Dutch ship in a storm by pouring oil into the sea.”
WAY of a Ship, is sometimes used for her wake or track. But more commonly the term is understood of the course or progress which she makes on the water under sail: thus, when she begins her motion, she is said to be under Way; when that motion increases, she is said to have fresh Way through the water; when she goes apace, they say she has a good Way; and the account of her rate of sailing by the log, they call, keeping an account of her Way. And because most ships are apt to fall a little to the leeward of their true course; it is customary, in casting up the log-board, to allow something for her leeward Way, or leeway. Hence also a ship is said to have head-Way, and stern-Way.
