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In the Irish Sea it was found that the place of the water at the half-tide interval did not correspond with that of a mark at the half-range of the vare, but that it was always below it, showing that the upper lialf of the wave rose and fell more rapidly than the lower. It was also found that the curve of the Irish Sea tide did not correspond with that of the Bristol Channel tide; that neither followed the law of the sines to corresponding arcs of tidal intervals.
In connexion with the range of tide is that of the apparent mean elevation of the water. All the observations confirm the remark of Professor Airy (Philosophical Transactions, 1845, Part I. p. 31), viz., that this mean level is higher at the springs than at the ncaps. The mean place of the water, however, for an entire lunation, during the summer months at least, is tolerably constant, and affords a fair standard to which the reductions used in our nautical surveys may be referred in the event of the gauge being remored by which the observations were made; annexed is the result of observations made at Holyhead during ncarly four entire years.
Apparent Meun Place of the Water, at Holyhead.
All the tides of the Irish Sea partake of the nature of river tides in haring their ebb longer than their flood, except those of Tuskar and Holyhead, which are the reverse. The respective intervals are given in the order in which the places occur.
Duration of Tide.
6. 8 Bardsey
6 52 Holyhead
6 0 Peel, Isle of Man 6 0
6 15 Ramsay, Isle of Man... 5 48
6 35 Fleetwood ....... 5 46
TIDES OF THE IRISH SEAS AND BRITISH CHANNELS. 217
All these are the mean of many observations.
to Ramsay, we shall see that Peel also, an intermediate station, is affected. The cause of this may possibly be connected with the effort of the water to maintain its level; for in projecting the curve of the wave on paper, this peculiarity, in connexion with the very short flood of Bardsey, has the effect of reducing the curve from what it would assume were Holyhead similarly influenced with other places.
It seemed evident that the water was influenced by forces acting in opposition nearly to each other, and that there was a tide in the offing whose streams of ebb and flood did not correspond with those of the channels. By applying this idea first to the English Channel, the observations responded to it;
and carrying it to the offing of the Irish Sea, and considering that channel as comprising the Bristol Channel within its limits, as the English Channel does the Gulf of St. Malo, the idea was confirmed so far as the observations themselves extended. This offing stream appears to be great extent, setting to the north and south along the coast of Biscay and the British Isles, running six hours nearly cach way, and exercising an influence with more or less effect over all the waters of the channels and estuaries it passes in its progress, diverting their courses, and in some cases, when the streams oppose, wholly overpowering or reversing their direction. From the connexion of the observations of the Irish Sea with those of the British Channel, it is clear that the whole of the ebb or outgoing stream of the eastern half of the Irish Channel runs into the Bristol Channel, and forms the flood or ingoing tide of the northern half of that great estuary; and vice versá the ebb or outgoing stream from the northern half of the Bristol Channel, forms the flood of the Irish Sea, each tide passing to and fro with great rapidity round St. Gowan's Head. The centre and southern half of the Bristol Channel receive their waters from the offing and the English Channel, the coast stream bringing the waters up from the Land's End and the English Channel, as the stream on the northern half did those of the Irish Channel, and vice versa.
The great offing stream at the entrance of the English Channel extends its influence as far up as Cape La Hague, beyond which, owing perhaps to the sudden contraction which there occurs in the Channel, the stream suffers no interruption, but, as in the Irish Sea, passes up and down the Channel six hours nearly each way as far as a line joining Dungeness and Cape Grisnez, the apparent virtual head of the tidal channel. Here the influence of the North Sea stream begins to be felt, and here, as in the Irish Channel, again the time of high and low water at the virtual head of the tide regulates the turn of the up and down streom along the whole channel as far as the contraction. Beyond this the offing stream being governed by its own high water, and that occurring at about six hours earlier than that of the head of the channel, the offing stream either butts against the returning streams from the channels, or withdrawing its water, solicits their streams and thus alters their course, making them for the most part set across the Channel in curves more or less bent as the spot is more or less removed from the offing ; so that there seems to be but one hour's tide each way that passes clean down the Channel from Beachy Head to Scilly, and round the Land's End to Bristol. The outgoing stream from Beachy Head encounters the ingoing stream of the offing tide somewhere about the Start Point, and both are turned down into the great Gulf of St. Malo, which seems to receive the accumulated waters of these opposite tides.
Whether or not this influx is instrumental in raising the water here to the extraordinary height of forty-seven feet perpendicular range at springs, or whether it be owing to its form and position as regards the advancing tide wave, is a problem; but it is a coincidence that cannot escape observation,
that this spot, like the Bristol Channel, is the concentration of streams from opposite directions; that it has its waters raised to the same extraordinary elevation nearly to a foot, and that its time of high water is nearly the same.
On the change of tide, this great bay, like the Bristol Channel, as it received so it returns its waters in opposite directions, the tide splitting somewhere between Alderney and the Start; but here especially, as also in a similar locality in the Irish Channel, we are in want of observations.
In tracing these streams, it was impossible not to be impressed with the many coincidences which assimilate the tidal phenomena of the two channels, so much so as to render it probable that they are subjected to precisely the same laws.
Considering the Irish Channel to extend from a line joining the Land's End and Cape Clear to the end of the tidal flow, which is either at Morecambe Bay or Peel, in the Isle of Man; and the English Channel as reaching from a line connecting Ushant with the Land's End, to the end of its tidal flow, or to Dungeness. We shall then see that the English Channel, from its outer limit to the end of its tidal stream, is 262 geographical miles, and that the Irish Channel, from its western limit to the end of its tidal stream, is nearly the same; being about 265 geographical miles. In both channels the stream enters from the south-west, and flows up until stopped by a counter stream. In both channels there is a contraction of the strait almost midway, by the promontories of Cape La Hague in one instance, and St. David's Ilead in the other, and at very nearly the same distances from the entrance. This contraction is, in both cases, the commencement of the regular stream, which flows six hours nearly each way, the turn of the stream throughout coinciding with the times of high and low water at the virtual head of the channel, situated in both cases about 145 miles above the contraction, and that time being very nearly the same, viz., 10h. 50m. at full and change; below this contraction, away from the land, the stream in both cases varies its direction nearly every hour, according to the force exerted upon it by the opposing offing stream.
In both cases, between the contraction and the southern horn of the channel, there is situated a deep estuary, the Bristol Channel and the Bay of St. Malo, in which the times of high water coincide, and where, in both cases, the opposing streams meeting in the channel pour their waters into these gulfs, and where the tides in both places rise to the extraordinary elevation of fortyseven feet at the syzygies. From the Land's End to the meeting of these streams in the Bristol Channel is seventy-five miles, and from Brest to the meeting of the streams off Guernsey the same. A still further coincidence is apparent between the phenomena of these channels. In one, at a place called Courtown, a little above the contraction of the strait, and at 150 miles from Cape Clear (its entrance), there is scarcely any rise or fall of the water; and in the other channel (about Swanage), situated also a little above the contraction of the strait, and just 150 miles from the Land's End, there is only five feet rise of the water at a spring range. In both cases these points of small range of tide are situated on the opposite side of the channel to that of the high elevation above-mentioned, and in both cases these spots are the node of the tide-wave (on either side of which the times of high and low water are reversed). And again we trace a similarity in an increased rise of the water on the south-east sides of both channels abreast of the virtual head of the tide: at Liverpool in one case, where the range amounts to thirty-two feet, and at Cayeux in the other, where it is thirty-four feet.
It may also be shown that the progress of the tide-wave along the side of the channels opposite the node is not very dissimilar. Reckoning in both cases from the line which we have before drawn, as the outer limits of the channel,
TIDES OF THE NORTH SEAS AND BRITISH CHANNELS. 219
and if complete
we find that in the English Channel, from this line to Cherbourg, opposite the small range of tide,
397 From Bardsey to Holyhead
193 From Holyhead to end of tide
959 Dieppe to the end of the tide
922 These numbers are given roughly, merely for the purpose of showing the general resemblance in the character and motion of the wave; and it is probable a more judicious selection of positions and numbers would give a still nearer coincidence. Besides which we are somewhat uncertain as to the establishment at our starting point. As a comparison, however, the numbers run fairly together. In both cases the retardation of the tide-wave about mid-channel
, and the great elongation of the wave towards the end of the strait are remarkable, especially in the Irish Sea.
Lastly, we may notice a singular coincidence in more respects than one, indeed, between the situation of the node placed by Professor Whewell in the North Sea, and a corresponding point of small range and inversion of tide at the back of Kintire. The node or hinge of the tide in the North Sea is curiously enough situated as nearly as possible at the same distance from the head of the tide off Dungeness, as the node at or near Suanage is on the opposite side of it; and the node at Kintire communicated by Captain Robinson, is about the same distance from the meeting of the tide in the Irish Sea as the North Sea node is from the meeting of the waters off Dungeness, and is similarly situated with respect to the node of Courtown as the North Sea node is with regard to Suunage. Report of further Obserrations upon the Tidal Streams of the North Sea and
English Channel, with Remarks upon the Lars by which those Streams appear to be governed.* By Captain Beechey, R.N. The method pursued in making these observations was to anchor the vessel at each of the stations for twelve hours and upwards, and to observe the direction of the tide, flood and ebb, every half hour. The rate of the stream, in addition to the usual method by the common log, was detected by current logs, constructed for the purpose by Mr. Massey, and which registered feet; and that it might be ascertained whether the stream was confined to the surface or extended to the depth of a vessel's hull in the water, another of these logs was occasionally sunk twelve feet, and registered simultaneously with the one which was two feet beneath the surface.
The time of slack water, as well as the times of the cessation and commencement of the stream, were noted as closely as such observations are capable of being made; and to render the times more certain, I have taken the mean in. terval between the time of the cessation of one stream and the time of commencement of the next.
Instead of these channels having, as has been hitherto supposed, a stream which turns progressively later as the tide advances up the strait, these observations have shown that the progressive changes of stream cease at a certain point near the mouth of the Channel, and that beyond that spot there is a tide peculiar to the channel, and quite distinct from that of the seas on either side of it; so that there may be said to be two distinct streams on each side of the Strait of Dover,-the stream of the Ocean or outer stream, and the stream of the Channel, or that which is contained between the Oceanic stream and the Strait of Dover. These streams are always running in contrary directions, and in the North Sea meet between the Texel and the estuary of Lynn; and in the English Channel between the Start and the Gulf of St. Malo.
* Philosophical Transactions, May, 1851.
In the locality where these streams meet, the tide is ever varying its direction according as the strength of one stream prevails over that of the other, giving to the water a rotatory motion, and scarcely admitting of any interval of slackwater, whilst in the spaces between these rotatory tides and the point of meeting of the tides in the Strait of Dover, the stream is free from all rotatory motion, and sets steadily throughout the tide in a direction towards Dover, while the water is rising there, and away from it while it is falling at that place.
I have designated the last-mentioned the true Channel stream, and its extent is, as nearly as it can be measured, 180 miles in either direction from the point of union of the tides in the Strait of Dover to the region of rotatory tides off Lynn, and off the Start and St. Malo.
As the true Channel streams are always running in opposite courses, there is necessarily a point where they meet and separate, and this occurs in the Strait of Dover ; but in this strait the stream, although it first obeys one tide and then the other, does not slack with the Channel streams, but is found to be still running at high and low water on the shore, at which time those streams are at rest, so that the Strait of Dover never las slack water throughiout its whole extent at any time. I have in consequence called this "an intermediate tide.” The limits of neither of these streams appear to be stationary, but runge
to and fro as the tide rises and falls at Dover, travelling to the eastwards on both sides, and at high and low water suddenly shifting sixty miles to the westward to re-conimence their easterly courses with the next tide; and although so far apart, they possess the remarkable peculiarity of shifting together; so that the Channel streams preserve as nearly as possible the same relative dimensions.
In the Strait of Dover this line of meeting and of separation oscillates between Beachy Head and the North Foreland, a distance of about sixty miles. When the water on the shore at Dover begins to fall, a separation of the Channel streams begins off Beachy Ilead. As the fall continues, this line creeps to the castward. At two hours after high water it has reached Hastings; at three hours, Rye; and thus it travels on until at low water, by the shore, it has nearly arrived at the North Foreland on one side of the strait, and at Dunkirk on the other. At this time the Channel streams on both sides slack, but in that portion which I call the "intermediate stream” in the Strait of Dover, the water is still running to the westward; and when the new channel streams make as the water rises on the shore, this intermediate point is found to unite with, or to oppose one or the other of these streams, according as it was before the reverse; so that, as before mentioned, the line of meeting at low water appears off Beachy Head to recommence its easterly course. This intermediate stream forms a remarkable feature in the tidal system of the Channel ;-it is well established, as the line of meeting and of separation occupies a very limited
space, and it seems to be entirely due to the contracted form of the Channel in this immediate locality, preventing the free escape of the water.
Captain Bullock, in order to test the point of separation, anchored two vessels a mile apart between Beachy Head and Dungeness, and found both vessels at the same moment to ride with their heads in opposite directions, in obedience to the streams which were then running opposite ways.