wave, the following experiments were made in the same channel filled to diffe

rent depths:

The former of these observations is exclusive of the height of the wave, and adding six inches to the depth of the fluid in this case, the height of the wave being already added to the depth in (12), we find that the velocities are nearly proportional to the square roots of the depths, and are nearly equal to the velocities that would be acquired by a heavy body in falling through heights equal to half the depth of the fluid.

In the last case the channel was rectangular, and consequently the depth of the fluid was uniform across the whole depth of the channel; it was next of importance to ascertain what law held in those cases where the depth diminished towards the edges of the channel. For this purpose two channels were selected having the greatest depths in their middle, and diminishing towards the sides. The following are the results:

In these instances the diminished depth at the sides has diminished the velocity of the wave below that due to the greatest depth in a ratio in the first example nearly of 95 to 7-8, and in the second of 7 to 6. See Experiments (11) and (12).

The following three experiments are instructive as having been made on channels in which the maximum depth was nearly the same in all; but in (15) the depth remained constant to the side which was vertical, in (16) the sides had a slope of nearly 200, and in (17) a slope of nearly 40°, so as to diminish the depth towards the sides.

From these it is manifest that the depth of the channel, while it modifies the depth of the fluid, affects the velocity of the wave. It was not found that the breadth of the channel produced any similar effect.

The report contained some experiments made on the river Clyde. The stations extended from the Bromielaw to Port Glasgow.

From a laborious discussion of the observations, it appeared that the wave of high water travelled

These results show that in the deep water being between 40 and 60 fathoms, or between 240 and 360 feet deep, the wave travels at the enormous rate of 80 miles an hour; that on reaching water from 20 to 30 feet deep, the velocity is diminished to 20 miles an hour; and from V. to II. where the river is wide, shelving, and shallow, the velocity of the tide wave is retarded to 8 miles an hour; while on ascending further up, where the banks are nearly upright, and the contracted width gives an increase of mean depth, the velocity has a corresponding increase to 15 miles an hour.

It appeared by the plans that the average depth of the river from I. to III. was 15 feet. From III. to V. the river is wide and shallow, spreading over extensive banks, where there are not 2 feet of water, for which we may take a third part of the greatest as a mean depth, or about 5 feet. In the division from V. to VII., both depth and breadth increase very rapidly to about 35 and 37; taking 25 feet as the mean depth, we have

The force of attraction having commenced its operations without having time, through change of position, to complete, a wave of the first order has been raised. This wave moves onwards over the sea, setting in motion the water before it, and leaving all still again behind. This wave is the great primary wave, raised at some distant period by the force of attraction on the sea, and moving onwards towards that point of the earth's surface the nearest to the moon, as directly as the obstructions formed by the land will admit of.

Professor Airy has calculated the original height of the tidal wave at two feet for spring tides, and nine inches for the neaps, which has been considered very nearly correct for the open sea ; but terrestrial influences, more particularly the bed of the sea as it approaches the land, give in the enlarged dimensions it attains on our shores.

Tides of the British Seas.-On this subject nothing can be

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more valuable than the observations of Admiral Beechey, and the following remarks are from his reports in the Philosophical Transactions, March, 1848, and May, 1851.

Tides of the Irish Sea.-Up to the period of these observations the set of the tides in the Irish Sea had been greatly misunderstood, owing to the association of the turn of the stream with the rise and fall of the water on the shore; and it was generally understood that as Holyhead was three hours later in its tides than places at the entrance of the channel, a vessel starting with the first of the flood would carry nine hours' tide in her favour in her run up channel, and vice versa.

This was an error sufficiently great in itself, but it was liable to be increased by an entire ignorance as to the time when a vessel would take any particular tide; for the times of high water by the shore were very various on different sides of the channel.

The observations have shown that, notwithstanding the variety of times of high water throughout the channel, the turn of the stream is simultaneous; that the northern and southern streams in both channels commence and end in all parts (practically speaking) at the same time, and that time happens to correspond with the time of high and low water on the shore at Morecambe Bay, an estuary rendered remarkable, as being the point where the opposite tides coming round the extremities of Ireland finally meet. So that it is necessary only to know the times of high and low water at Morecambe Bay to determine the hour when the stream of either tide will commence or terminate.

By these observations,

Taken in connexion with the very valuable series of observations which were carried round Ireland by the Ordnance at the suggestion of Professor Airy, we are made acquainted with several curious facts:-First, that whilst it is high water at one end of the channel, it is low water at the other; that the same stream makes both high and low water at the same time; that there are two spots in the channel, in one of which the stream runs with considerable velocity without the water either rising or falling, and in the other, that the water rises and falls from 16 to 20 feet without having any visible horizontal motion of its surface; and that during the first half of the flowing, and last half of the ebbing tide-wave, the stream in the south channel runs in a contrary direction to the wave, and goes up an ascent of about one foot in four and a-half miles.

Plate II. shows the lines of direction of the stream with the rate of the tide at its greatest velocity on the day of syzygy, all being reduced to the same standard.

An inspection of the Plate will show that the tide enters the Irish Sea by two channels; of which Carnsore Point and Pembroke are the limits of the southern one, and Rathlin and the Mull of Kintire the boundaries of the northern.

The stream in the southern channel (as before stated) has been ascertained to move simultaneously in one vast current throughout; running six hours

*For plans, see Admiral Beechey's Charts in the Phil. Trans.; in the absence of these, consult the Admiralty or the Ordnance maps.

nearly each way, at an average rate of from two to three knots per hour at the height of the springs, increasing to four knots and upwards near the banks and at the pitch of the headlands; its times of slack water corresponding sufficiently near for all practical purposes, with the times of high and low water for the day at Morecambe Bay, or more correctly at Fleetwood, which is twelve minutes earlier than Liverpool.

The central portion of the stream of flood or ingoing stream runs nearly in a line from a point midway between the Tuskar and the Bishops, to one six miles due west of Holyhead; beyond which it begins to expand eastward and westward, but its main body preserves its direction straight forward for the Calf of Man, which it passes to the eastward with increased velocity as far as Langness Point, and then at a more moderate rate on towards Maughold Head. Here it is arrested by the flood or southern stream from the north channel coming round the Point of Ayre, and is first swayed round to the eastward by it, and then goes on with it at an easy rate direct for Morecambe Bay.

The outer portions of the stream are necessarily deflected from the course of the great body of the water by the impediments of banks on the Irish side of the Channel, and by the tortuous form of the coast on the Welsh. The eastern portion passing Linney Head rushes with great rapidity between the Smalls, Grassholm, and Milford Haven, towards the Bishops, which it passes at a rate of between four and five knots; sets sharply round those rocks in an E.N.E. direction, right over the Bass bank, and into Cardigan Bay; makes the circuit of that bay; and sets out again towards Bardsey at the other extremity of it; then sweeping to the N. by W. past the island and through the sound, it gradually takes the course of the shore, round Caernarvon Bay, filling the Menai Strait as far as Bangor; but the stream still continuing outside towards the South Stack, which it rounds, setting towards the Skerries at a rate of upwards of four knots; and finally, turns sharply round those rocks for Liverpool and Morecambe Bay; completing in its way the high water in the Menai, and filling the Dee, Mersey, and Ribble.

The western portion of the stream, after passing the Saltee, runs nearly in the direction of the Tuskar, sets sharply round it, and then takes a N.E. N. direction, setting fair along the coast, but over the banks skirting the shore. Abreast of the Arklow is situated that remarkable spot in the Irish Channel, where the tide neither rises nor falls. The stream, notwithstanding, sweeps past it at the rate of four knots at the springs, and reaches the parallel of Wicklow Head. Here it encounters an extensive bank recently known; and whilst the outer portion takes the circuit of the bank, the inner sweeps over it, occasioning an overfall and strong rippling all round the edge, by which the bank may generally be discovered; beyond this point the streams unite and flow on towards Howth and Lambay, growing gradually weaker as they proceed, until they ultimately expend themselves in a large space of still water situated between the Isle of Man and Carlingford, where occurs the phenomenon of the water rising and falling without having any perceptible stream. This space of still water is marked by a bottom of blue mud.

In the north channel the stream enters between the Mull of Kintire and Rathlin simultaneously with that passing the Tuskar into the southern channel, but flows in the contrary direction. It runs at the rate of three knots at the springs, increasing to five knots near the Mull, and to four near Torr Head on the opposite side of the channel. The eastern branch of this stream turns round the Mull towards Ailsa and the Clyde, a portion passing round Sanda up Kilbrannin Sound and Loch. Fyne.

The main body sweeps to S. by E., taking nearly the general direction of

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the channel, but pressing more heavily on the Wigtownshire eoast; off which it has scooped out a remarkable ditch, upwards of twenty miles long by about a mile only in width, in which the depth is from 400 to 600 feet greater than that of the general level of the bottom about it. Near the Mull of Galloway the stream increases in velocity to five knots, the eastern portion turns sharply round the promontory towards the Solway, and splits off St. Bee's Head; one portion running up the Solway, and the other towards Morecambe Bay.

The central portion from a midway between the Mull of Galloway and the Copeland Islands, presses on towards the northern half of the Isle of Man, and while one portion of it flows toward the Point of Ayre, the other makes for Contrary Head, and is there turned back at a right angle nearly to its early course. Passing Jurby it reunites with the other portion of the stream, and they jointly rush with a rapidity of from four to five knots round the Point of Ayre, and directly across all the banks lying off there, and catching up the stream from the south channel off Maughold Head, they hurry on together towards that great point of union, Morecambe Bay. This bay, the grand receptacle of the streams from both channels, is notorious for its huge banks of sand heaped up in terrible array against the mariner unacquainted with its locality, and also remarkable for a deep channel scoured out by the stream, and known as the Lune Deep, which, to the wary navigator, is the great hidden beacon of his safety, and serves him, alike in fog or in sunshine, as a guide to his position, and to a harbour of safety in case of need.

We have now only to speak of the western limit of the stream, which we left off Torr Head running at a rate of four knots off the pitch of the Point. Hence it strikes directly towards the Maidens, boiling over the Highlander and Russell rocks, and other reefs in the vicinity of that dangerous group; and takes the direction of the coast again from Muck Island to Black Head, at the entrance of the Lough of Belfast, which it fills.

The portion of the stream which sets up the Lough splits again off Grey Point; one portion flowing up towards Garmoyle, while the other bends back along the shore of Bangor, Grimsport, and Orlock, and blends with the general stream which has come on from the Maidens and Black Head, and passes with it through the sounds of the Copeland Islands. Hence it proceeds along the coast, brushes the South rock, and runs on towards St. John's Point; off which, the stream, like that coming from the southward, expends itself in a large space of still water, which remains undisturbed although pressed upon by streams from various quarters.

Such is a general description of the streams in both channels which attend the flowing of the water, or which, for the purpose of distinction, we may designate the ingoing stream.

The ebbing or outgoing streams do not materially differ from the reverse of these, except that in the southern channel they press rather more over towards the Irish coast.

This is a general idea of the course of the streams throughout the Irish Sea, represented in Plate II.; but besides these there are (as usual) at all the points and headlands, when abrupt, counter streams or eddies beginning at about two hours after the offing stream, increasing with the strength of the tide, and occasioning races and overfalls at the places marked on the chart. In the direction of the offing stream there is as little variation of the current at the different hours of tide as will be met with in any sea of similar extent, and indeed it is only with the slackening of the tide that the variations occur, which happens from about forty minutes before to about forty minutes after high or low water at Morecambe Bay.