HANWAY, 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
Harbours. 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 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.
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 casu. 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,
thrown 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 Barrahead, 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. read off. 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 = the greatest force that can assail a pier, = height of waves which produce (after being corrected for obliquity) the maximum effect, and which are due to the line of maximum effective exposure. = sine of azimuthal angle formed between directions of pier and line of maximum effective exposure, radius being unity. Then 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 .
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 Wigtownshire, 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 waves. 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 Stroma 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 further 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 9½ 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 (Instit. 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.