Jonas, a social reformer and philanthropist of the last century, was born in 1712 at Portsmouth. He served his apprenticeship to a merchant in Lisbon, and in 1743 became partner in an English firm in St Petersburg. His business led him to travel into Persia, and on his return he published An Historical Account of the British Trade over the Caspian Sea, &c., in 4 vols. 4to—a work of no high literary aims, but of great practical use to the mercantile men of the day. The work had great success; and Hanway, encouraged by the result, continued for the remainder of his life to use his pen, though chiefly for the sake of the many charitable and philanthropic schemes which he either set on foot himself or took a strong interest in. He founded the Marine Society and the Magdalen Charity, both still in existence, and in their respective spheres doing much good; and strenuously promoted the Sunday-schools, then in their infancy. His great services were at length to meet their reward. In 1762 a deputation from leading merchants of London was successful in obtaining for him from government a commission-ship of the navy. The name of Hanway often occurs in the social history of these times. He was a handsome man, and knew that he was so; indeed at St Petersburg he used to be called "Le bel Anglais." He took great care of his person, and on one occasion became the talk of the town for carrying about an umbrella with him; and as that curious engine did not come into vogue till about thirty years later, its first supporter had much ridicule and banter to encounter. His whims on the subject of tea are well known. Hanway died September 5, 1785.

Harbours

Are either natural or artificial. Some parts of the British coasts are amply provided with natural bays and creeks, while in other parts the accommodation and shelter for shipping have been entirely supplied by artificial means. Thus, Ireland and the west coast of Scotland are plentifully intersected by excellent deep water bays and anchorages; but on the east and south-west shores of Britain there are but few natural harbours. Cromarty Bay is 200 miles distant from the Firth of Forth, which is the nearest southern natural harbour; while there are no less than 400 miles between the Firth of Forth and the Thames, which may be considered as the next really unexceptionable harbour of refuge. On the west coast there are about 200 miles of coast between the nearest natural harbours of Holyhead and Loch Ryan. The construction of artificial places of refuge becomes therefore a very important matter in a country where every winter's lists of shipwrecks and loss of life, remind us how much nature has left for art to accomplish. For the most complete body of evidence regarding the ports of Britain, we cannot do better than refer to the volumes of Reports by the Tidal Harbour's Commission, for the completeness of which the public is mainly indebted to the zeal of Captain Washington, the present indefatigable Hydrographer to the Admiralty.

The designing of harbours constitutes confessedly one of the most difficult branches of civil engineering. In making such designs, the engineer, in order to avail himself of the advantage which is to be derived from past experience, must endeavour to the best of his power to institute a comparison between the given locality and some other, which he supposes to be in pari materia. Perfect identity, however, in the physical peculiarities of different stations, seldom if ever exists, and all that can be done is to select an existing harbour, which appears to be as nearly as possible similarly circumstanced to the proposed work.

In considering the subject of the construction of harbours in exposed situations, the first and most important subject deserving our attention is the destructive action of the element with which we have to deal,—what are its energies when excited by storms, and what the direction of its forces on the barriers which have been raised to control it?

Smeaton, in his history of the Eddystone, when speaking of the objection that might be raised against the necessity for using joggles in the masonry of that building, says, "When we have to do with, and to endeavour to control those powers of nature that are subject to no calculation, I trust it will be deemed prudent not to omit in such a case anything that can without difficulty be applied, and that would be likely to add to the security." This statement of our greatest marine engineer, indicates the propriety of carefully collecting any facts that may help us to a more accurate estimation of those forces which he regarded as being "subject to no calculation." We shall therefore state a few facts which have been recorded of the destructive power of the waves in inland lakes, and in the open ocean.

The writer has seen at Port Sonachan, in Loch Awe, where the fetch is under 14 miles, a stone weighing a quarter of a ton, torn out of the masonry of the landing slip and overturned. Mr D. Stevenson, in his Engineering of North America, describes the harbours in Lake Erie as reminding him of those on our own sea-girt shores, and mentions having seen one stone weighing upwards of half a ton which had been taken out of its bed in the pier at Buffalo, moved several feet and overturned. The Comte de Marsilli, in his Histoire Physique de la Mer, published at Amsterdam in 1725, states that the highest wave observed by him on the shores of Languedoc in the Mediterranean Sea, where the breadth is about 600 miles, was 14 feet 10 inches. At the mouth of a harbour on the German Ocean, with a fetch of about 600 miles, the writer had observed for him the height of the waves during south-easterly gales, and on one occasion the result was 13½ feet from the crest of the wave to the trough of the sea. In deeper water, and with a north-easterly gale there is no doubt that the waves of the German Ocean will attain a height considerably greater than this. In November 1817 the waves of the German Ocean overturned, just after it had been finished, a column of freestone 36 feet high and 17 feet base. The diameter at the place of fracture was about 11 feet. In the Atlantic Ocean, Dr Scoresby stated, in a communication to the British Association in 1850, that during several hard gales he had measured many waves of about 30 feet, but the highest was 43 feet from the hollow to the crest. Waves of such magnitude could scarcely, however, reach our artificial harbours from the shallowness of the water near the shore. To these facts it may be added, that we know (from the testimony of an eye-witness) of a block of 50 tons weight being moved by the sea at Barra-head, one of the Hebrides; and what is far more extraordinary, we know, and can vouch for the fact, that blocks of 9 tons weight have been quarried, or broken out of their beds in situ, on the top of the Bound Skerry of Whalsey in Zetland, which is elevated 85 feet above the level of the sea. The Bound Skerry and neighbouring rocks, which are in the German Ocean, certainly furnish by far the most wonderful proof that has yet been discovered, of the great force which is developed by the billows of the ocean when suddenly checked by opposing rocks.

The writer has stated (in the Trans. Roy. Soc. Edinburgh) that, from the observations which he had made with the marine dynamometer (a self-registering instrument designed by him for the purpose), he had found the force of the waves of the German Ocean during hard gales, to be 1½ ton per superficial foot at the Bell Rock, and of the Atlantic Ocean to be 3 tons per superficial foot at the Skerryvore Lighthouse. But these results may still be far short of the maxima. As the marine dynamometer has been often found useful in indicating the force of the waves in situations where harbours were to be built, it may be proper to give such a description of it as will enable any one to have it made.

DEFD is a cast-iron cylinder, which is firmly bolted at the projecting flanges G, to the rock where the experiments are to be made. This cylinder has a circular flange at D. L is a door which is opened when the observation is to be Harbours. A is a circular disc on which the waves impinge. Fastened to the disc are four guide rods B, which pass through a circular plate C, which is screwed down to the flange D, and also through holes in the bottom EF. Within the cylinder there is attached to the plate C a powerful steel spring, to the other or free end of which is fastened the small circular plate K, which again is secured to the guide rods B. There are also rings of leather T, which slide on the guide rods and serve as indices for registering how far the rods have been pushed through the holes in the bottom, or, in other words, how far the spring has been drawn out by the action of the waves against the disc A.

In comparing an existing harbour with a proposed one, in order to ascertain the dimensions which are necessary to insure stability, perhaps the most obvious element is what may be termed the line of maximum exposure, or, in other words, the line of greatest fetch or reach of open sea, which can be easily measured from a chart. But though possessed of this information, the engineer still does not know in what ratio the height of the wave increases in relation to any given increase in the line of exposure.

As this inquiry is one of great moment in the practice of marine engineering, and has not been in any way investigated, the writer has for some time back been making occasional observations on the subject, when favourable circumstances occurred. These observations have been but limited in extent, and cannot be regarded as deserving of confidence unless in cases where the two harbours are not far different in their lines of exposure. So far as these experiments have gone, the waves seem to increase in height most nearly in the ratio of the square root of their distances from the windward shore.

It does not follow, however, that the line of maximum exposure is in every case the line of maximum effective force of the waves; for this must depend not only on the length of reach, but also on the angle of incidence of the waves on the walls of the harbour. What may be termed the line of maximum effective exposure is that which, after being corrected for obliquity of impact of the waves, produces the maximum result, and this can only be taken from the chart after successive trials. Let \( x \) = the greatest force that can assail a pier, \( h \) = height of waves which produce (after being corrected for obliquity) the maximum effect, and which are due to the line of maximum effective exposure. \( \sin \alpha = \sin \alpha \) of azimuthal angle formed between directions of pier and line of maximum effective exposure, radius being unity. Then \( x \propto h \sin^2 \alpha \) when the force is resolved normal to the line of the pier; but if the force is resolved again in the direction of the waves themselves, the expression becomes \( x \propto h \sin^3 \alpha \).

It should not be forgotten, in connection with this subject, that there are various qualifying elements to which special attention requires in some cases to be given. The waves, for example, may often be noticed, when approaching the land obliquely, to alter their direction when they get close to the shore (in consequence of the depth changing), so as to strike it more nearly at right angles to the general line of the beach. In this way a swell from the ocean may enter a bay which is not directly exposed to it. It should also be observed, that the lines of exposure cannot be directly compared if the depths of the water through which they pass are materially different.

The tides, too, exert in many places a very decided effect on the nature of the billows, in some places causing waves of an unusually dangerous character, while at others they are found to run down the sea. If a marine work is situated in a race or rapid tide-way, such, for example, as those called "roosts" in Orkney and Shetland, the masonry will be exposed to the action of a very trying and dangerous high-cresting sea. As an example of this, we may refer to Port-Patrick in Wigtonshire, where the violence of the waves is, we have no doubt, much due to the rapidity of the tides. If, on the other hand, the race or roost runs in such a direction as to be entirely outside of the harbour, and at some distance off, it will have a decided tendency to shelter the works, and to act as a breakwater. Thus it appears, from observations specially made for the writer at Sumburgh Head Lighthouse in Shetland during a south-westerly storm, that so long as the Sumburgh roost (one of the most formidable in those seas) was cresting and breaking heavily, one could have easily landed in a small boat at a creek or bay called the West Voe; but no sooner did the roost disappear towards high water than there came in towering billows that totally submerged cliffs of very considerable height.

The study of the modifying and intensifying effects of tide-currents on the waves of our British seas seems to have been entirely neglected in the late discussions regarding the merits of vertical and sloping walls, which will be referred to in another section of this article.

We think it right to mention that we consider as erroneous the opinion expressed by a writer in the Edinburgh Philosophical Journal—that the cause of races or roosts is the meeting of two rapid currents; neither do we believe that they are occasioned by the projection of rocks from the bottom of the sea as many sailors suppose.

From careful inquiries, as well as from actual personal experience, of such formidable breaking waters as the Boar of Duncansbay, and the Merry Men of Mey in the Pentland Firth, and several others, we are of opinion that the true cause is the swell of the sea encountering a tidal current running in a direction more or less opposed to that of the swells. While it is obvious that two rapid tides may meet each other without any dangerous effects, it is also quite true that when two tides meet each other in a rough sea, as in coming round such islands as Stronsa or Swona in the Pentland Firth, the effect of their union being to increase the current at that place, there will be produced a highly dangerous sea; but the fact of their meeting, though calculated to aggravate, is not, we think, the primary cause. The races which occur in open seas, as, for instance, off headlands and turning-points of the coast, are certain portions of those seas in which the waves break to a greater or less extent, although the water may be very deep, and there may be no wind at the time. At all such places it will be found that there are rapid tides. The roosts on the west coast of Orkney or of the Pentland Firth, for example, are worst with ebb tides and westerly swells, because the Atlantic swell and current of ebb are opposed. Those again on the east coast are worst with flood tides and easterly swells from... Harbours, a similar cause. Thus at the east end of the Pentland Firth the Boar of Duncansbay is well known to rage with easterly swells and a flood tide; whereas, at the west end of the same firth, the Merry Men of Mey are equally well known to be worst with ebb tide and a westerly swell, at which time no boat could enter them without the risk of being overturned. The dangerous surf which exists at the mouths of some rivers is, we believe, not solely due to the want of depth at the bar, but also to the meeting of the outward current with the waves of the sea.

When a swell encounters a rapid opposing current, the onward motion of the waves seems to be arrested, and their width becomes visibly decreased. They get higher and steeper, crest, and at last break, sometimes very partially, and at other times almost as they would on a shelving beach. It appears to us possible that several waves may ultimately combine in such disturbed waters into one mountainous billow; for the wave that has partially broken may have its onward motion so much checked as to allow the wave behind to overtake it, and having thus coalesced, they may, as one large wave, acquire a superior velocity, so as to overtake those in front, and be farther augmented by the union of other waves which have been reflected from the shore.

It is to this cause we are inclined to refer such wonderful effects as that to which we have already alluded, where blocks of 9 tons weight were quarried out of the solid rock at an elevation of 85 feet above the sea. Were such violent action common to all the shores of the German Ocean, instead of being restricted to one or two similar places, half of our eastern seaport towns would, without any doubt, be washed into the sea during the first stormy winter. As a further proof of the great effect of the tides in exasperating the waves, we may mention that the time when most damage is done to sea-works which are in tolerably deep water, is from one to two hours before and after high water, which nearly corresponds to the time when the tide runs strongest outside. We have found this to hold true at many different parts of the coast, but will only refer to one well-marked instance. At Peterhead harbour, which projects prominently into the sea on an isthmus, the tides, at but a short distance seaward of the harbour, run very rapidly. On the 10th January 1849 there was a very heavy sea, and a crowd of people were down, about two hours before high water, helping to secure the whalers and other vessels in the harbour, when three successive waves carried away 315 feet of a bulwark, founded 93 feet above high-water springs, which had stood for many years. One piece of this wall, weighing 13 tons, was moved 50 feet. After this outbreak of the sea the waves became more moderate, until about two hours after high water, by which time the large whalers had taken the ground, when other three enormous waves again swept over the harbour, submerging the quays to the depth of from 6 to 7 feet of solid water, by which sixteen people were drowned. These waves filled the harbour to such a depth as to set all the whalers afloat again, and they continued so for several minutes, until the excess of water had run out through the harbour mouth.

These gigantic waves were, in our opinion, clearly the result of some such action as has been attempted to be described. We should not have dwelt at such length on this subject were it not that we might again refer to the facts when we come to treat of the subject of vertical and sloping walls for harbours of refuge, where it is of importance to show that even in the deepest water, the waves are not purely oscillatory, but that wherever there is a tide-way the waves will more or less partake of the qualities of waves of translation.

Another circumstance affecting the exposure of any marine work is the depth of water in front of it. The great mountainous billows so commonly met with in the Atlantic Ocean cannot be generated in the shallower waters of the German Ocean, unless perhaps in such peculiar circumstances as have just been adverted to. It becomes, therefore, of great consequence to ascertain the maximum possible wave in a given depth of water.

Mr Scott Russell, whose observations on what may be called the marine branch of hydrodynamics are of such great value, has stated that if waves be propagated in a channel whose depth diminishes uniformly, the waves will break when their height above the surface of the level fluid becomes equal to the depth at the bottom below the surface (p. 425 Brit. Assoc. Rep. on Waves). This statement, the meaning of which seems doubtful, Mr Russell elsewhere (Insti. Civ. Eng., p. 136) defines thus: "The author has never noticed a wave so much as 10 feet high in 10 feet water, nor so much as 20 feet high in 20 feet water, nor 30 feet high in 5 fathoms water; but he has seen waves approach very nearly to those limits." It is presumed that the datum here referred to is the mean level of the surface of the sea. We have had no opportunities of verifying these observations; but as the subject is very important—because the depth of water in front of a work may be said to be the ruling element which determines the amount of force which it has to resist, whatever be the line of maximum exposure, we shall simply state what has come within our own knowledge and observation. We have repeatedly seen at different parts of the coast breaking waves of from 4 to 5 feet, measuring from hollow to crest, in from 7 feet 8 inches to 10 or 11 feet of water, measuring from the bottom up to the mean level; and on one occasion we were told of waves which were estimated at 9½ feet in 13 feet water. It must, however, be borne in mind that these observations, and we conceive also those of Mr Russell, apply only to common waves of the sea, or those short, steep, and superficial waves which are due to an existing wind, and not to the ground swells which are almost constantly to be found in the open ocean, and which may be the result of former gales, or are the telegraph, as Mr Russell terms them, of those which are yet to come.

From what has been stated, it would appear that in most cases the heaviest waves should assail any tide-work at high water. This, however, as mentioned in the last section, is not always the case, the greatest damage being often found to occur at the time when the tide runs strongest.

Mr Leslie found that the Arbroath Harbour-works were in general less severely tried by the very heaviest waves than by a class of waves somewhat smaller than these, owing to the outlying rocks, which, from the small depth over them, had the effect of tripping up the heavier seas, and thus destroying them before they reached the harbour, while the depth was sufficient to allow the smaller waves to pass over the shoals unbroken. In some cases of severe exposure the waves might to some extent be reduced by dropping very large stones outside of the harbour, so as, by forming an artificial shoal, to cause them to crest and break.

One great difficulty connected with the subject of the generation of waves still remains unsolved, viz.—What are the minimum line of exposure and area of sea which are compatible with the existence of a ground swell? This question, we fear, cannot be answered in the present state of our knowledge.

Deep Water Harbours.

Harbours of refuge are distinguished from tidal harbours mainly by the superior depth of water which they possess, and the larger area which they inclose. The requisites of harbours shelter during storms, and easy access for shipping at any time of tide. There has been much discussion as to whether piers for harbours of refuge should be vertical or sloping. Col. Jones, R.E., has especially advocated the superior merits of the vertical wall; and the discussions on his plan at the Institution of Civil Engineers, and the able The principle which is asserted is, that oceanic waves in deep water are purely oscillatory, and would occasion no impact against vertical barriers, which would be the most eligible, as they would only have to encounter the simple hydrostatic pressure due to the height of the advancing billow, and would reflect the waves without causing them to break.

Were it even admitted that the waves were purely oscillatory, and were reflected by a vertical barrier, would no force, it may be asked, be expended when the motion of the particles was reversed? The reflection of a wave is equivalent to the nearly instantaneous creation of a wave in the opposite direction, for which a very considerable force must surely be required.

We believe, however, that from the effect of tide currents, to which we have already referred, and perhaps from other causes whose action seems to have been overlooked by the advocates of the upright wall, any form of barrier, in whatever depth it may have been erected, must be occasionally subjected to heavy impact. We conceive that the possibility of waves of translation being generated in the deepest water has been already established, if we succeeded in satisfying the reader of the truth of the following assertions:—First, That waves break in deep water during calm weather; a fact which is apparent to the eye and familiar to all sailors; and, secondly, and negatively, That to leeward of those races or portions of broken water, which certainly do not reflect the incoming waves, there is comparatively smooth water both at sea and on the adjoining shores, until such time as the strength of the tide is exhausted, and the roost has disappeared, when violent action is again fully manifested.

It may be argued that these are extreme cases, and that such high velocities in the current of the tide are seldom met with. This objection has, no doubt, truth in it; but still the tendency is shown, and though the velocities may be less in other quarters, there may yet be quite enough to destroy the condition of stagnation which the oscillatory theory assumes. The breaking of waves at sea, and the existence of races, seem to prove beyond question that waves of translation are possible in the deepest water. Is it not also a probable case that waves which have been reflected by a vertical wall, and have (irrespective of the question of tide currents) combined with the advancing waves, may then become waves of translation, possessing all the elements which endanger the stability of a sea work? Or, again, how much more damage would result to a vertical wall than to a slope of loose stones, from the sinking of the foundations, or from their getting underwashed by the reaction of the waves? It therefore appears that the method generally resorted to of forming deep water harbours of masses of rubble stone with long slopes, so as to form an artificial beach for the waves to spend on, is, in most circumstances, the best and cheapest kind of construction. We incline, however, to the adoption of an upright wall, founded on the rubble as a basis (similar to that at Cherbourg, about to be described), in preference to long paved slopes, as there is always experienced a great difficulty in founding the toe of such talus walls among the loose rubble. When pitched slopes are adopted, great benefit will be found to accrue from leaving at the bottom or toe of the slope a wide foreshore. Much, however, depends on local peculiarities in selecting the best design for any work; and the nature of the bottom is all-important. Where the bottom is soft, a vertical wall can hardly, if ever, be attempted.

In making these remarks, we must not be understood as condemning the adoption of vertical walls in cases where the foundation is good. All that we assert is the opinion, that waves of translation do exist in deep water, and therefore that harbours of refuge will prove failures unless they are built in such a manner as to resist the impact of those waves of translation. The Cherbourg breakwater has often been referred to as a successful instance of the application of a vertical wall, and has been contrasted with the Plymouth breakwater, which has a long slope. But this appeal is quite fallacious, as the profile of that work is, as already hinted, of a composite character, consisting of a talus wall sloping at the rate of 10 horizontal to 1 perpendicular, surmounted by a plumb wall; so that whatever merit may be supposed to belong to the vertical profile is entirely nullified at Cherbourg by the long talus wall in front, on which the violence of the waves is much broken. Moreover, the heaviest waves at Cherbourg come from the N.W., and do not assail the breakwater at right angles to its direction, but come more nearly end on to the work, so as to a great extent to run along the outer wall. The N.W. waves are propagated from the Atlantic, while the waves which are most trying to the work come from the N., in which direction the line of exposure is only about 21 leagues. These facts we obtained during a recent visit to Cherbourg, undertaken for the special purpose of ascertaining the physical characteristics of the place. The attempt to make out a parallelism between Plymouth, which faces the Atlantic directly, and Cherbourg, which is comparatively land-locked, cannot, in our opinion, stand the test of a candid inquiry.

Other comparisons may be referred to which have been advanced on equally untenable grounds. Thus, the old pier of Dunleary, which is vertical, and has stood well, has been compared with the talus walls of Kingstown Harbour, which now protect Dunleary, and which have often received much damage. The all-important element of depth of water has been in this instance entirely overlooked; for at Kingstown there is a depth of 27 feet, while Dunleary is all but dry. An able writer on the same questio vexata, in comparing different sea walls in the Firth of Forth, has, in like manner, not sufficiently adverted to the great differences in the depths opposite the works to which he refers.

An important advantage of the sloping wall is the small resistance which it offers to the impinging wave, but it should also be borne in mind that the weight resting on the face stones in a talus wall is decreased in proportion to the sine of the angle of the slope. If we suppose the waves which assail a sloping wall to act in the horizontal plane, their direct impulse, when resolved into the force acting at right angles to the sloping surface of the talus wall, will be proportional to the sine of the angle of incidence. The effective force when estimated in the horizontal plane, will be proportional to the square of the sine of the angle of incidence. But if we assume the motion of the impinging particles to be horizontal, the number of them which will be intercepted by the sloping surface will be also reduced in the ratio of the sine of the angle of incidence, or of elevation of the talus wall. Hence the tendency of the waves to produce horizontal displacement of the wall, on the assumption that the direction of the impinging particles is horizontal, will be proportional to the cube of the sine of angle of elevation of the wall.

If it farther happens that there is obliquity of action in the azimuthal as well as in the vertical plane arising from the relative direction of the pier and of the waves, there will be another similar reduction in the ratio of the squares or cubes of the angle of incidence according as the force is resolved into that at right angles to the line of the pier, or to that of the direction of the waves.

Let $\phi$ = vertical angle of incidence or angle of elevation of wall; $\phi'$ = azimuthal angle of incidence; $f$ = horizontal force exerted on unit of surface at right angles to the line of harbour wall; $h$ = height of greatest assailing waves; $f' \propto h (\sin \phi \sin \phi')$. The above expression assigns, we think, too great a reduction, as the motion of the particles may not be horizontal, and no account is taken of the effects of friction against the rough surface of the masonry. Experiments are therefore wanting to determine the constant for correcting the theoretical results due to this expression. For further information on this subject, we refer the reader to the article on HYDRODYNAMICS.

Mr Scott Russell recommends the parabolic curve as that best suited for the profile where the object is to break the waves, and not to reflect them, as is the case in sloping breakwaters. This curve possesses, according to Mr Russell, the advantages of superior strength, of economy in the materials, of breaking the wave early, and of continuing an uniform action over the longest period of time. When the tide is low, the toe of the slope, which springs out of the foreshore and forms the vertex of the parabola, would, we fear, be found rather weak, and perhaps difficult to form. On the whole, we rather incline in such cases simply to throw in the materials, and to allow the sea to form its own slope.

According to Sir John Rennie (Account of Plymouth Breakwater), rubble breakwaters with slopes formed at the angle of repose, were adopted by the Greeks in the moles of Tyre and Carthage, and by the Romans at Athens and Halicarnassus. The same design was also followed at Venice, Genoa, Rochelle, Barcelona, and other places. In this kingdom the first example on a large scale which we find is at Howth, Kingstown, Holyhead, and the noble breakwater at Plymouth, were afterwards carried out on the same principle, and chiefly under the directions of the late Mr Rennie. The great national harbours of refuge at present in progress in this country, according to Mr Rendel's designs, at Holyhead and Portland, are on a similar principle; while those under Messrs Walker, Burgess, & Cooper, at Dover, Alderney, and Jersey, are more nearly vertical.

On the best Forms of Walls for Tidal Harbours.

Having now considered the few facts of which we are in possession regarding the disputed nature of the impulse of the waves in deep waters, we shall direct the reader's attention to their effects in shallow water. Those in deep water were chiefly whole waves, and regarded by many as being purely oscillatory, while those in shoal waters are breaking waves, and therefore regarded by all as waves of translation. We have hitherto been considering breakwaters erected in deep water, and which were constantly exposed to the waves; we now turn to piers and sea-walls which are placed within the range of the surf, and which are exposed to its force for a limited period only, being sometimes left nearly, or altogether dry by the receding tide.

The impulse of the waves against a sea-wall or pier may be resolved practically into four directions—1st, The direct horizontal force which tends to shake loose, or carry before it, the blocks of which the opposing masonry consists. This force may also blow up the pitching, or overturn the inner or quay-wall by condensing the air, or pressing upon the water which occupies the interstices of the rubble. We know two cases in the German Ocean where, in consequence of want of width in the pier, coupled, in one instance, with insufficient workmanship, the inner or quay-walls were observed first to bulge and fall, before the sea-wall was injured. One of these piers measured 26 ft. 4 in., and the other 24 ft., on the roadway. 2d, The vertical upward force which may act on any projecting stone or protuberance. 3d, The vertical downward force of the water which results either from the wave breaking upon the toe of a talus wall, or from the wave passing over the parapet, and falling upon the pitching behind, so as to plough it up. 4th, The backdraught which tends by reaction from the wall to plough up the soft bottom, and thus to undermine the lower courses of the work, or perhaps by suction to pull out the facework. We may conclude from the above that the points which require to be carefully attended to are—1st, The contour and quality of masonry of the wall itself; 2d, The parapet, which, if not of sufficient height, or built in a proper direction, leads to damage in the pitching behind it; and 3d, The foundation-courses, in the design and construction of which, if similar precautions be not attended to, underwashing of the bottom may in some situations take place, so as to leave the lowest courses without protection.

We shall in the first place consider how far those remarks are applicable where the bottom is solid rock. Such a supposition will render unnecessary any precautions arising from the wasting of the bottom, and, ceteris paribus, there does not seem to be any reason for preferring a talus to a vertical wall. The question of preference in such a case will in the main depend upon the kind of material which can be obtained. Should the stone be scarce or costly, and the quality such as to warrant the introduction of masonry of the best description, the vertical wall may be found to be the most economical. Where freestone is to be used, it is not only desirable that it should be got in large blocks, but that the face stones should possess considerable hardness. This precaution is particularly necessary in selecting the stones for the lower courses, and especially where the beach consists of hard gravel. For the same reason, it is highly important that all stones which are subject to decay from atmospheric influence should be either entirely rejected or assembled in the upper courses of the parapet.

Where the materials are abundant, but of an unworkable nature, a long talus wall will generally be found most economical. For such walls the rate of slope must depend very much upon the exposure of the place, and upon the plentifulness of rubble-stone hearth. The easily-dressed and naturally flat-bedded materials, which the stratified rocks of the secondary formation very often furnish, are especially applicable for the construction of vertical walls; while the uncouth blocks of the primary and igneous formations are better suited for talus walls. Such rocks as gneiss, the schists, basalts, greenstones, amygdaloids, and the tougher kinds of granite, are best fitted for this purpose. With some of those rocks the angularity of the pieces, and the excessive difficulty of dressing, render it necessary to assemble them without almost any alteration of their shape, by an adaptation of their salient and re-entrant angles, so as to make a kind of random rubble face-work. In this kind of work, mortar is very seldom employed. The parapet generally consists of squared masonry, surmounted by a heavy coping, and it should in every case be set in good lime mortar.

Where the materials are light and of small sizes it is desirable to equalize the action of the sea over the whole work, and not to concentrate it against any particular place. Mr Russell states that the cycloidal form was recommended for this purpose by Franz Gerstner of Bohemia. The only instance with which we are acquainted of the adoption of this curve was in a sea-wall erected at Trinity, near Edinburgh, by the late Mr Robert Stevenson, in 1822.

It has been already stated that, irrespective of the quality of the masonry, the two points in the structure which are on soft or weak or dangerous are the top and bottom of the wall. With a rocky bottom the risk of failure at the foundations is removed; on the other hand, where the shore consists of rotten rock, moving shingle or sand, it is obvious that provision must be made for both those sources of evil. In fact, if we consult the history of harbours, we shall find that by far the most frequent cause of damage is the reaction of the sea against the shore.

The general slope of a fragmentary beach must depend upon the size and nature of the particles and the force of the sea. The dissimilarity between the slopes of a beach near the levels of high and low water, arises from a decrease Harbours.

in the force of the waves, owing to their being broken before they reach the high-water mark. The great object, therefore, is to design the profile of our wall so as to alter as little as possible the symmetry of the beach. Where isolated rocks or large boulders are seen projecting above the surface of a sandy beach, there will generally be formed around them hollows, corresponding in depth to the kind of obstruction which the rocks present. The principal point in the design, therefore, must be to avoid great and sudden obstructions to the movement of the water. The best form which could be adopted in any situation would of course be the same as the cross section of the beach itself, but this would answer no possible purpose; and, as the wall is to consist of heavy blocks of stone instead of minute particles of sand, it is clear that a much steeper slope may be adopted than the profile of conservancy of the coast, provided the lower part of the slope be flattened out so as to meet the sand at a low angle. The action of a bulwark is to arrest the waves before they reach the general high water mark, and to change the horizontal motion of the fluid particles to the vertical plane, or to compel the waves to destroy themselves on an artificial beach consisting of heavy stones. To prevent underwashing, the two following requisites should therefore be as far as possible secured:—1st, The foundation courses or bottom of the wall should rise at a very small angle with the beach, so that their top surfaces may be coincident with the profile of conservation of that portion of the beach out of which the wall springs; 2d, The outline of the wall should be such as to allow the wave to pass onwards without any sudden check till it shall have reached the strongest part of the wall, which should be as far from the foundation as possible.

Those two requisites show clearly how inapplicable a vertical wall must in most cases be for a sandy beach. Instead of altering the direction of the wave at a distance from its foundation, the whole change is produced at that very point, and unless the wall be founded at a great depth, its destruction is all but certain. Where the materials are costly, but admit of being easily dressed, we are disposed to think that horizontal, or nearly horizontal wall connected with a vertical one by a quadrant of a circle may be found suitable. Such a form will prevent to a considerable extent the danger of reaction by causing an alteration in the form of the wave at that part where the wall is strongest and at the greatest distance from the toe or curb course. Where the materials are abundant and of a rougher nature, a cycloidal wall, with vertical and horizontal tangents somewhat similar to that erected at Trinity, to which we have already referred, may be adopted with advantage.

A special caution may not be out of place regarding clayey bottoms. Many are apt to suppose that there can be no better foundation than clay; and it is indeed true that some kinds of hard clay form a satisfactory subsoil. But there are others of a softer kind, and permeated by sandy beds, which are extremely treacherous. If there be the slightest dip seawards, there is always a risk of any pier that may be built on such a base slipping bodily into the sea. This holds especially true of inland lochs, where the sides very often slope suddenly. In one instance, the particulars of which we got on the spot shortly after the accident, a pier built on a clayey beach, sloping below low water at the rate of 1 in 12½, suddenly began to move, and after two hours it had slipped seawards 150 feet, and had by that time descended bodily a height of 34 feet, the top of the pier being then no less than 23 feet below low-water spring tides.

Construction of Harbours.

Our space will not admit of our going much farther into the subject of the construction of harbours than the few remarks we have already made. A knowledge of such matters may to some extent be acquired by a careful perusal of the published histories of marine works; but, after all, it must be confessed that the only valuable teacher in this wide practical field is experience. It is, in truth, impossible to lay down any general rules of guidance as to matters of this kind. All that can be done within our space is to notice very briefly some of the more important methods of working. And first, with regard to that invaluable piece of apparatus, the diving-bell, we would refer to the article on the subject Diving in this work; and to Smeaton's Account of Ramsgate Harbour, published in 1721, where it was first applied by him to harbour works. The diving-helmet is a most useful and convenient modification of the diving-bell, and is now very generally employed.

Of late years Mr Walker has introduced from France the Beton use of beton as a substitute for backing. This artificial concrete is sometimes used in enormous masses. We have seen at Cherbourg blocks of 50 tons prepared in boxes, whose sides and tops are removed after the concrete has set, in order to be again similarly employed. The proportions used at Cherbourg by M. Rebeille were two of sand or fine gravel, to one of Portland cement.

We may also mention that the method of assembling stones Edge work, on their edges, instead of on their beds, which formerly was in use in some old Scottish harbours and sea-walls, as at St Andrews, Prestonpans, &c., deserves to be more generally known and adopted, from its superior strength.

The proposal of Mr Bremner, of Wick, for putting in Mr Bremer proposes to construct, in some adjoining place the foundations of low-water piers also merits notice. Mr Bremner proposes to construct, in some adjoining place of shelter, enormous pontoons of timber, on which the under parts of the work are built, and afterwards floated to the desired spot in favourable weather, and carefully grounded. Such a plan might, we have no doubt, be found economical and useful in some situations.

Mr Rendel has introduced an improved method of assembling the pierres perdues or rubble used in the construction of large breakwaters; this method he employed at Millbay Pier, near Plymouth, in 1838, in a depth of 38 feet; and he is at present carrying out the same principle on a still larger scale, in the construction of the breakwaters at Holyhead and Portland. The improvement consists in depositing the rough materials from stagings of timber elevated a considerable height above high water. The stones are brought on the staging in waggons, through the bottoms of which they are discharged into the sea. The principle on which the stagings are designed is that of offering the smallest possible resistance to the sea, the under structure consisting of nothing more than single upright piles, there being only one line of piles for each roadway.

Mr Rendel, in a letter kindly communicated to us, states, "I use no timber braces of any kind, as these offer more resistance to the sea than strength to the staging. At Portland and Holyhead, however, where any accident would be a serious evil, owing to our employing convicts in the quarries, we stay the piles with iron guys, fixed to Mitchell's screw moorings, and also truss the outer piles in each row with iron rods. We also fix the piles in the ground with a screw."

"At Holyhead, however, we only attach to each pile boxes filled with small stones, for the purpose of getting them into a vertical position, and use no stays or guys of any kind."

"The superstructure consists simply of balks of timber, with rails laid on them to carry the waggons. The piles are placed in rows 30 feet apart, and the ease and certainty with which the staging is constructed is such that a length of 30 feet, including the screwing in of the piles, the laying down of the roadways, and all minor works necessary to make them fit to carry the waggons, never occupies more than one working day and a half, and often less. The length of the piles that we are now using varies from 84 to 90 feet, the depth of water at both Holyhead and Portland being about 11 fathoms." Harbours. "Of the strength of the stage you may judge from its carrying on each roadway as much as three waggons, weighing in the gross 12 tons each.

"The advantages of the staging are obvious. It contributes greatly to the consolidation of the stone, it makes a greater length of breakwater to be under construction at the same time, and it enables the deposits to be carried on without interruption, almost in the heaviest weather. As an instance of this, I may remark that my resident at Portland informs me that the waggons and locomotives were engaged yesterday at a time when such a sea was running that large bodies of spray were thrown 55 feet above the water level. As a proof of the facilities which the stage affords for rapidity of construction, I should state that we have deposited this year at Holyhead, where free labour is employed, nearly one million tons of stones. The loss from accidents to the stage is comparatively small on its first cost, and when spread over the cost of the whole works it is a mere trifle. I find the sea-slopes are, in the deep water and exposed parts, from 5½ to 6 to 1 between 6 feet above high-water and from 12 to 15 below low-water, from which point they rapidly become about 1 to 1. The inside slopes are never more than 1½ to 1, and seldom more than 1 to 1. The materials are excellent for our purpose."

Mr Walker has also kindly contributed some facts connected with the construction of the great works now going on under the direction of Messrs Walker, Burgess, and Cooper at Jersey, Alderney, and Dover. At Alderney, which is a very exposed place, the base, up to 12 feet below low-water, is formed by stones thrown, or rather dropped in from barges. Up to low-water the work is all done by diving-helmets. The wall is faced with granite, backed with blocks of beton made of sand, shingle, and Portland cement. Above low-water it is faced with stone of the island, a kind of millstone-grit, and is backed with blocks of rubble set in Roman cement. The millstone-grit is raised in very large blocks. The profile is to consist of a quay, an esplanade, and a parapet.

Jersey is much the same as Alderney, but the pell-mell work is carried to low-water, having nearly vertical walls of conglomerate built above. Dover has nearly vertical walls, faced with granite from the very bottom, which is now 45 feet below low-water. This work was done with diving-bells.

Sir J. Rennie, in his Account of the Plymouth Breakwater, says, "From the bottom to within 8 feet of low-water springs, we find that the slope is 2½ or 3 to 1. Here the effect of the waves is comparatively small, being neutralized by the mass of water. From thence to low-water of spring-tides the slope increases from 3 or 4 to 1, but between low-water of spring-tides and high-water, when the effect of the waves is greatest, there we found that the rubble would not lie at less than 5 to 1, whilst, on the inside, the slope stands generally at from 1½ or 2 to 1."

The above interesting details regarding these national works show, from the variety which they exhibit, how difficult it is to lay down any general rules for the construction of harbours, and confirm the principle that each work must be judged of per se.

Miscellaneous Observations.

The ultimate object of constructing harbours is, by lowering the height of the waves, to preserve the tranquillity of the area of water which is inclosed by the piers; and this property is variously possessed by harbours of different forms, and depends much upon the relative widths of the entrance, and of the interior, the depth of water, the shape of the entrance, and the relation between the direction of its opening, and that of the line of maximum exposure.

The only formula of which we are aware is that by the writer of this article (Edin. New Phil. Journal, 1853), which gives an approximation to the reductive power, or is, in other words, a numerical form of expressing how much a wave of given height becomes reduced, after it has entered a harbour. Though the results obtained by the formula may not be absolutely correct, this will be no objection where the object is merely to obtain a comparative value, as, for example, in comparing one design for a harbour with another.

When the piers are high enough to screen the inner area from the wind, where the depth is uniform, the width of entrance not very great in comparison with the width of the wave, and when the quay walls are vertical, and the distance not less than 50 feet,—let

\[ H = \text{height in feet of waves in the open sea} \] \[ x = \text{reduced height of waves in feet at place of observation in the interior of the harbour} \] \[ b = \text{breadth of entrance to harbour in feet} \] \[ B = \text{breadth of harbour at place of observation in feet} \] \[ D = \text{distance from mouth of harbour to place of observation in feet} \]

\[ x = H \left( \sqrt{\frac{b}{B}} - \frac{1}{50} \left( 1 + \sqrt{\frac{b}{B}} \right) \sqrt{D} \right) \]

This formula has been found to give good approximations at several harbours where the heights of the waves were registered. When \( H \) is assumed as unity, \( x \) will represent the reductive power of the harbour.

In situations where the highest waves cross the harbour mouth at an oblique angle, a farther reduction is due to height of this cause. We have been unable to find any observations that have been made on this subject by others, and lateral deflection, for want of better, we shall give three observations made under our directions at Latheronwheel harbour:

| Angle of obliquity | Height of wave run through | Height of wave run through angle | |-------------------|---------------------------|---------------------------------| | 0° | 16 feet | 1'00 | | 50° | 32 ... | 0'68 | | 140° | 68 ... | 0'21 |

These must, however, be regarded as but approximations. It is obvious that as the wave may be deflected through more than 360°, the curve representing the reduction must be a spiral; but more observations are wanted to determine what kind.

Booms are logs of timber placed across the mouth of a harbour or the entrance to an inner basin or dock, having their ends secured by projecting into grooves cut in the masonry on each side of the entrance. The booms are dropped into those grooves to the number of from 10 to 20, or as many more as will insure close contact of the lowest one with a sill-piece placed in the bottom of the harbour, without which precaution the swell is found to enter the harbour from below the booms. By this contrivance, which forms a temporary wall, the waves are completely checked and prevented from spreading into the interior basin. The longest booms we have seen are about 45 feet, and in some places, as at Hartlepool and Seaham in Durhamshire, they are taken out and in by steam power.

Though perfectly successful in their tranquillizing effect (provided they are kept in contact with the sill piece at the bottom), booms are not suited for the mouths of harbours where there is much traffic, as the shipping and unshipping of so many logs of timber can hardly take less than a quarter of an hour—a delay which might in many cases be attended with serious consequences.

It is very desirable, and in some cases essential, that there be either a considerable internal area, or else a separate basin opposite the entrance for the waves to destroy or spend themselves. Such a basin should, if possible, be made so as to preserve a portion of the original shore for the waves to break upon, and when circumstances render this impossible, there should at least be a flat talus of 2 or 3 to 1. Talus walls of 1 to 1, or steeper, will not allow the waves to break fully, but will reflect them in such Harbours.