AN INVESTIGATION OF THE TIDES BETWEEN THE OWERS AND PORTLAND, FROM OBSERVATIONS MADE DURING The progress of the SURVEY ON THE SOUTH COAST OF ENGLAND.-By Captain Sheringham, R.N., Admiralty Surveyor.

THE following paper on the marginal tidal streams of the English Channel by Capt. Sheringham, R.N., whose zeal and intelligence peculiarly fit him for prosecuting such an enquiry, is a valuable contribution to hydrography. A correct knowledge of the inshore set of the Channel tides was required for completing the examination of its tidal streams, which had been so successfully commenced by Capt. Beechey, R.N., the result of whose enquiries has been communicated to the Royal Society in two papers of considerable interest; one of which we have already noticed in our number for February 1849, and the other we hope shortly to present to our readers. As Capt. Sheringham, has adverted to the subject, we may here briefly state that these papers advocate a system of tidal streams peculiar to channels, in which a combined wave is formed by the meeting of two tidal waves from opposite directions, such as Capt. Beechey has shewn to be the case in the English Channel and in the Irish Sea. In channels so circumstanced it is found that the turn of the stream, instead of being progressive as was hitherto supposed, takes place nearly simultaneously throughout, and that the times of slack water coincide nearly with the times of high and low water at the apex NO. 8.-VOL. XX.

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of the combined wave, or as it has been otherwise called the virtual head of the tide. This theory has greatly simplified the question of the streams of our channels, and will no doubt be thankfully received by the navigator. Numerous carefully conducted observations have confirmed the correctness of the system, and Capt. Sheringham's bear witness to the same effect.-Ed. N.M.

To Admiral Sir Francis Beaufort, K.C.B., Hydrographer to the Admiralty.

SIR.-I cannot conceive a more interesting or a more useful study than a careful investigation of the phenomena of the tides and their effects on our own shores; and as it appears to you that local knowledge combined with a connected series of observations is indispensable in order to grapple effectually with the subject, I have been induced at your desire to undertake their discussion on that portion of the coast, the survey of which has been confided to my care by the Lords Commissioners of the Admiralty.

am fully impressed with the difficulty of the subject, and the danger of being carried away by crude theories, and I am equally aware of my want of ability to cope with it. But, nevertheless, apart from the consideration that there is always something new to be learned even from the most humble labourer, I feel it is the duty of every honest officer, not to excuse himself by a false pride, from giving to the world the result of labours purchased at his country's expense.

The admirable paper on the tides in the Irish Sea, &c., published by Capt. Beechey so fully deserves the attention of every enquiring mind, that I can scarcely convey an idea of the pleasure and instruction I received from its perusal; and I may add that it gave no small impulse to the determination which induced me to undertake my humble share in the enquiry.

The paper in question has brought to light many important facts, some of which, that officer will be gratified to hear explain certain phenomena in our local tides, to account for which, has heretofore baffled the endeavours of many.

Asking your excuse for these preliminary observations I will at once enter upon my subject.

To render more intelligible the following remarks I have annexed an explanatory plan, diagrams, &c., the symbols on which, and the rules adopted in their construction should be explained.

On the plan the arrows shew the directions of the stream at the different quarters of the tide where they appear, as actually observed, the feathered indicating the flood, and the plain arrows the ebb, according to the rise and fall of the tide in Portsmouth harbour. It is manifest in order to shew where the tidal streams cross, merge, or deflect each other, that the observations, should be simultaneous. But as in practice, that is unattainable, it was therefore necessary to refer them to one time and place, and Portsmouth Harbour has been selected for that purpose.

The expressions flood, or ebb, are used according as it was a rising or

falling tide at the guage, and the dots on the arrows denote the quarters of the tide.

To avoid a repetition of the plan and to shew the probable progress of the tidal wave, deduced from the observations on it, the continuous line on it indicates the eastern or in-coming stream, and the broken line the western or out-going stream.

The hours inserted in Roman figures shew the time that the stream turns where they stand on the days of syzygy, and those marked on the land express the establishment of the port and the range of an ordinary spring and neap tide.

A few of the greatest velocities are introduced in the lines of direction for the purpose of shewing where the wave acquires or loses its force. The limits also of the shoals of three fathoms water, and ten fathoms water are defined by dotted lines.

Captain Beechey has stated that, in every part of the English and Irish Channels, that is to say, in their navigable portion, the important fact may be accepted for all practical purposes, that the time of slack water is simultaneous, and that time he has assumed to be at XI and V on the days of syzygy.

As far as my observations have gone, I can confirm this statement; and where discrepancies have been found, they are either of small amount or to be accounted for by some local influence.-For example:

Outside the Owers half a mile the stream turns at

One mile east of Bembridge

Five miles east of Dunnose

Three miles south of St. Catherines
Two miles south of St. Albans Head
Near the east end of the Shambles

Travelling in one wave about six hours each way.

It may be conceded even if we had not conclusive evidence of the fact, that the intervention of islands, promontories, and shoals, and the reflux tide, even from small estuaries, would have a material effect upon the set and velocity of the tides, not only by deflecting them from their natural course, but in some instances completely reversing their direction. This fact is manifested in a very remarkable manner, in the tides at Spithead, and along the adjacent coast, producing phenomena which although familiar enough to those locally acquainted, as far as I know have not yet been explained.

Lieut. McKenzie, the old surveyor, who must have been a shrewd and close observer, appears to have been forcibly struck with the peculiarities in the set of the tides inside the Isle of Wight, and infers that they are not to be accounted for.

In treating of the rise and fall of the tide in Portsmouth Harbour, he says; " "The first four hours flood rises only about three inches more than the three last hours, and it is equally remarkable, that the two last hours ebb fall about four feet three inches more than the three first hours ebb, (vide his table following) by which it appears that the

hourly rise or fall is by no means in proportion to the time, and baffles all common calculation."

It is shewn then by the above table which I find to be very nearly correct, (see the diagram of ranges,) that contrary to the usual laws of ebb and flow in rivers, the flood tide in Portsmouth Harbour is of longer duration than the ebb, by about two hours, as indeed is more or less the case at Southampton, Langston, Chichester, and all the harbours in the Solent, &c. The cause of this as well as the unequal rise and fall may be thus accounted for.

It is to be observed that with the exception of Chichester and Southampton, there is little or no back water to affect, prolong, or increase the volume of the ebb, and therefore, for the sake of argument we may treat the range and duration of the tide as the simple effect of tidal

water.

We find at Portsmouth that for the first four hours of the flood the water rises pretty equally but slowly, altogether seven feet three inches, which seems to be supplied by that portion of the in-coming wave, which splits off the Needles, and flows up the Solent through Spithead. But we prove from actual observation, that four hours after the in-coming tide has made in the offing, or about nine o'clock on full and change days, the western stream has made in-shore the whole way from Selsea Bill to the westward, (see plan). This stream unites with the last of the flood round the island and runs back again through Spithead, giving increased effect to the gradual swelling of the channel or in-coming stream, filling all the different harbours in its progress to the westward, not only in an increased proportion, but prolonging its duration until the stream has turned in the offing a little after eleven o'clock, making a flood, or rising tide of seven hours.

The unequal fall of the ebb also may be thus explained.

It is to be presumed that if no local cause existed to overrule the natural fall of the tide, it would do so soon after high water by the shore, as the out-going stream had already made in the offing; indeed we know this to be the case, for at St. Helens and Selsea Bill, there is a regular rise and fall of six hours. But it is reasonable to suppose

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that as long as the ebb tide maintains its strength through Spithead, the water would be pressed back or prevented from falling out of the harbour as quickly as it otherwise would, and so we find that the check lasts until the Spithead tide has slacked, when both Southampton and Portsmouth rapidly empty themselves, causing the tide to fall at the accelerated rate which is shewn by the table and diagram.

The ebb has a still greater influence over the rise and fall of the tide in the little harbours of the Solent and to the westward. We will take Yarmouth as an example, the establishment of which may be called X and III. At ten o'clock the tide slacks at the Needles, at which time it is also high water at Yarmouth; the tide on shore is then observed to fall from four to six inches, but about half-past ten o'clock the ebb has acquired considerable strength down the Solent, which is no sooner the case than the tide is forced up again and rises to the same level as before making a second high water at about twelve o'clock, when, as the channel ebb or out-going stream has made nearly an hour, the tide begins to fall regularly until low water between three and four o'clock, about the same time as the stream slacks near the shore.

Before we follow the effect of the ebb tide along the coast to the westward, where similar phenomena, within certain limits are observed, I will endeavour to explain more clearly, why the last half of the ebb runs to the eastward at Spithead while it is setting to the westward elsewhere.

To return to McKenzie we find him to say," It is further worthy of remark, that whereas the duration of the south-east stream at Spithead is two hours longer than the duration of the north-west stream there, yet in the channel between Old Castle Point and the Bramble the stream runs as long to the north-westward as it does to the southeastward. I mention this remarkable circumstance without being able to account for it, as I believe it impossible."

The truth of the above is proved by many observations, which may be seen and readily understood by referring to the plan; but let us again state the fact in words.

At Spithead or any where between the Bramble and the Horse Sand the eastern stream runs seven hours, and the western stream five hours, that is to say there is a strong counter tide running from Southampton Water to Bembridge for nearly two hours; see the broken line on plan. This fact has puzzled people amazingly, but it appears to me that the simple reason is this. As soon as the western stream at Spithead has slacked, the rapid discharge from Southampton Water and Portsmouth Harbour produces such an increased velocity in the back water as absolutely to turn or beat back from its course the languid stream through Spithead, converting the ebb or out-going into an in-coming tide, maintaining its superiority until the channel widens between the Horse and Bembridge Point, some where near the Nab, where it meets the regular channel ebb, their confluent streams sweeping round to the southward of the Isle of Wight. Thus the ebb tide for the last half of its duration runs in opposite directions on the two sides of the eastern half of the island.