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Harbour and Marine Surveying. - Preliminary Observa
tions.--Outline of Coast.–Starring out.-Buoys.-Station Pointer.—Stations.—Buoying out Harbour.—Triangulation.-Selection of Base. --Advantages of the System.Triangulation Station Poles.--Instrumental Observations off the Coast.- Soundings.—The Hand-lead and the Plunger:-Holding Ground.—Anchoring.—Tidal Observations.—Tides of the Irish Seas.—British Channel and North Sea.—Mean Tide Level.-Indian Ocean.-Tide Gauges.
Amongst the principal requirements of a harbour, the following conditions will have to be considered, and obtained as nearly as circumstances will allow.
The access should be easy for a disabled ship to enter during violent gales, and there should be facilities for working out of harbour with contrary winds.
The harbour should afford shelter and security for vessels which otherwise must be driven on a lee shore, but in harbour they should ride at anchor in safety, and without danger of being driven against each other.
There should be a considerable area of deep water for large ships, and the greater the depth the better, because some accumulation is inevitable in a barbour where the water is quiet.
There should be good holding ground for anchorage, that is, there should be sufficient tenacity in the ground at the bottom of the harbour to bring a ship up which may enter in a gale of wind.
The nature of the shore should be such as to offer beaching ground, to allow of crippled vessels being hauled up thereon and secured.
Breakwaters should be so disposed that as little as possible of injurious effects should result by the depth of water being decreased.
These conditions and general rules—and it must be remembered that there is no rule without an exception-will be met more or less by the natural advantages of the spot selected, by works to be constructed, by prevailing winds, the nature of the bottom, depth of water, tides and currents, &c., which will be the objects of research in the survey, or of future study when it is completed.
The depth of water will have to be ascertained by numerous soundings, all of which will have to be reduced to ordinary low water spring tides, and the depths figured on the plan will indicate the depths below this low water level. The exact situation at which these soundings are taken will have to be ascertained by observations made with angular instruments, in order that such situations may be correctly shown on the plan; and as necessarily these must be very numerous, some very ready and practical means must be found to effect the object.
It will also be necessary to compute the areas enclosed at different depths, that is, how many acres there will be of water at 2, 3, 4, 5, or 6 fathoms, &c., as the case may be. These will be calculated from the plan, so that unless the soundings are correctly entered on it, the computations must be incorrect.
As the nature of the ground all over the bottom of the harbour will have to be ascertained, instrumental observations will have to be made also wherever the investigation is made ; for inasmuch as we may in different places find clay, gravel, sand, rock, or boulders, &c., the sites of these must be correctly shown on the plan, also the extent of area over which these deposits are spread; their depth, if merely thinly superficial; whether they be natural or merely accidental deposits; if a rock bottom, whether it be the natural or geological formation, or whether it may not be simply an accumulation of stones accidentally imbedded, The tenacity of the holding ground will have to be investigated—what portions may be good, bad, or indifferent, with the areas occupied by each ; what portions may be improved, as for instance, by breaking up, or by removing boulders or lumps of rock, or by other means; where moorings may be required, &c.; in short, the most careful attention will have to be given also to this subject, for its complete investigation will generally be more or less of vital importance to the project.
Wherever works of construction are proposed, more particularly if they will require to be of a large or expensive description, an examination of the materials on the spot, or in the immediate vicinity, will be of the greatest importance; for instance, whether there be shingle and sand for making concrete, or blocks of “ béton,”* or rock for the construction of breakwaters, piers,
Béton" be said to be French for concrete, though somewhat differently manufactured; it is made into large blocks, and lowered when hard into the spot where it may be required.
quay-walls, &c. It will be very necessary to examine very closely into the nature and construction of such rock, as to whether it be granite, sand, or limestone ; whether the rock be
; of a laminated or homogeneous description; how it bears the action of waves or batter of the sea; how it will blast or quarry; the average size of the blocks; what ashlar and what rubble may be produced from it—of what hardness and density; the depth of baring before the rock is reached; as close an approximation of the cost of the stone as can be obtained ; also what use may be made of materials at hand, what quantity will have to be brought to the works, from where, and by what means. Where the works are to be of a heavy nature, these details will all prove very important elements in the engineer's estimate.
The sets of the tides will have to be investigated, and often the currents; as also the rise of ordinary, as well as of the highest and lowest spring tides, and of neaps.
A few observations as to the tides will be required for those who have never paid any attention to this subject. The periodical rise and fall of the sea known as the flood” and “ebb” of the tide, are due to the combined attractions of the sun and the moon; the tide rises and falls twice during the return of the moon to the meridian, and on the coast generally takes about as much time rising as falling.
The tides though constant are not equal, but greatest at the time of the new and full moon, and least between these periods, or greatest when the moon is in conjunction or opposition to the sun, and least when in quadrature to it. The tides of greatest rise are called spring tides, and those of least rise are called
The lowest as well as the highest water occurs at the spring tides, but it does not rise so high, nor fall so low, at the neap tides; and lastly, the greatest spring tides are those which occur at the time of the equinoxes, after the autumnal, and before the vernal. The spring tides do not happen precisely at the new and full moon, nor the neap at the quarters, but a day or two after.
Some idea of the movement of the tidal wave may be formed from the circumstance of the tide being considered to occupy twenty-four hours in travelling from the southernmost point of Ireland to the Thames; this amount of time being occupied in passing round to the north of Scotland, and coming round by the German Ocean to enter the Thames. A portion of the same tidal wave, on presenting itself on the south-west of our coast, diverts off into the Channel, and, proceeding in this course, joins the other tidal wave near Dover.
The regularity which would be the result of the earth being covered with deep sea is disturbed by the great shoals, narrow straits, positions of land, breadth and depth of channels. Further observations will be made on this subject towards the end of the chapter.
The regularity of all the tidal phenomena at different places is very greatly affected by storms and particular winds; so much so that neap tides sometimes rise as high and higher than springs, and this is more particularly the case on any part of the coast where there is less difference in the rise of springs and neaps. A high wind coinciding with “flood,” will cause it to rise much beyond the height due to that particular tide, and the reverse of this may occur if the wind blows hard in an opposite direction during the rise of the tide. On the opposite coast of France and England, a strong south wind produces an excess in the rise of the tide on our side, and the reverse on the opposite coast.
In tidal rivers the anomalies are still more remarkable, and the rise of the tide is affected very sensibly by the banks, their contractions and bends, as well as by currents and land-waters.
Altogether these anomalies and irregularities are so considerable, that practical observations and gaugings are indispensable, and require to be carried on with the greatest attention.
The methods of investigating all these matters, and of conducting the surveys, will now become the subjects of the following remarks :
One of the first things to be done will be to obtain charts of prior dates, if there are any, to compare them together, and make notes of discrepancies between them, and this not so much with regard to differences as to the outline of the coast—such as little projections and indentations, but as regards depths of water and bottom of harbour ; for should any such exist, they will bave to be carefully examined ; and as regards depth, not so much as regards one isolated depth, but as to a continuous line, for this would lead us to iufer either silting up the formation of shoals, or the water becoming deepened. This comparison of charts will have to be all the more carefully made accordingly as they inay otherwise the better agree; lines should, therefore, be carefully laid down on the charts according to their respective scales, and the measurements should be taken from the scales shown on the charts themselves; on each map, a line or two between prominent objects should be laid down, when a few triangles based upon these will enable us to lay down the line of difference between the two charts. These triangles should be so laid down
OUTLINE OF COAST.
by means of the sides oearing on distinct points on land, so that they may be accurately traced on the water. These subjects are of much greater importance than they appear at first; but it will afterwards be found in the course of the survey that a good chart is of the greatest value.
The next thing to be done will be to obtain local information, which is generally to be best procured from old resident fishermen,-as to depths of water, surface of bottom, tides, currents, formation of shoals, &c. A great deal of valuable knowledge as to the matter in hand is to be obtained from them.
So much of practical detail on surveying generally has been given in previous chapters; and with regard to bases, and
Surveying by Observation”in the last chapter, that we shall not have to go over that ground again; but inasmuch as we know by experience how strong is the predilection of taking up the proctical subjects, and neglecting what appears purely theoretical, we must warn the reader that unless he has made himself acquainted with the contents of the last chapter, he will feel at a loss in reading the following remarks.
The outline of the coast should be surveyed from a system of triangulation off a base, measured or computed from another base as already shown; and the position of this base must entirely depend not only on the form of outline, but on the facilities for obtaining a correct measurement, and on the position of objects on the coast, inland and out at sea. We infer, however, that from previous study the reader bas made himself master of this portion of the subject; but we must now observe, as regards the triangles thrown up from the base, that the sides of these must be so laid down that as many as possible of the stations of the main triangles, as well as of stations fixed by observation along the sides, should be so placed as to be seen from seaward, so that from thence observations may be made upon them; and to this leading feature particular attention must be given. It is far more on account of these latter observations that a correct triangulation has to be laid down, than on account of every little bight or indentation of the coast line; for a minor error as regards the position of a station as to the coast line itself, would probably become greatly magnified when observed upon from a distance in the offing
Two of the most troublesome cases likely to arise are where a town surrounds a harbour, or where a coast is bounded more or less by a bold and lofty cliff, the base of which is washed by the waves at every spring tide; in the first the system of triangulation we require is not practicable, and in the second the stations on