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BUOYING OUT HARBOUR.--NOTE-BOOK.
at a, for b and c, with only this difference, that we shall have to operate with 500 instead of 2000, and 1000 instead of 1500. To buoy out the lines into the offing from a boat at b and c, set out with a sextant carefully adjusted at 90°, as b, e, f, and c,g, h; and similarly with the other lines. Or from B we may set out Ba', perpendicular to BC, and from a', a' C, perpendicular to Ba', and measure off on a' C' the lengths required, and so determine the lines wanted. There will now be no difficulty in perceiving that on the line 6, 6', e, f, we may take with the sextant as many angles as we may require soundings, B 16, B2b, B36, &c., the boat being kept carefully on the line buoyed out, either by means of the buoys or the stations on shore. The stations determined by the angles thus measured may be laid down on the plan by means of the station pointer in the manner already explained ; for the line b, e, f being set off on the plan in the same manner we have just supposed being done in the offing, it is only necessary to lay off the angle B 1 b on the limb of the instrument, and making one bar coincide with b, e, f, move the instrument along the line, until the other bar coincides with the station B, when the centre of the instrument is over the point required. We consider, however, that it may be much more readily done by keeping the note-book in the following manner, and computing the distances b1, 62, 63, be, &c.
NO. OF LINE-STATIONS OBSERVED-LENGTH OF
RADIUS = 2000 FEET.
Angles Complementary Tabular Distance measured. Angles. Tangent. in Feet. 76° 46' 13° 14' •2351
470 (=b1) 61° 51' 28°9' -5351 1070 (=b2)
62 56° 5' 33° 55' .6724 1345 (=63) 49° 2' 40° 58' •8682 : 1736
42° 32' 47° 28' 1.0900 2180 ( = b 4) The first measured angle is taken at 1; the three angles of the triangle B 1b being equal to two right angles, one of which is at b, the remaining two angles are equal to the remaining right angle, and if from 90° we subtract the observed angle, the remainder will be the complementary angle 1 B b, to which b 1 is the tangent, B b being radius; at I the observed angle is 76° 46', which, subtracted from 90°, leaves the complementary angle 13° 14' = ] Bb; the tabular tangent of 13° 14' = -2351, which multiplied by radius B b = 2000 = 470 feet = b 1. Proceeding in the same manner with the next angle 61° 51', observed at 2, we shall find 2 = 1070 feet; and b 3 = 1345 feet; and b c = 1736, and b 4 = 2180, &c.; these distances being all plotted by scale from the point b, determine the position of the points or stations required. We may proceed in the above manner with any number of lines which are at right angles to another, as L, K, k, l, m, n, or any others. To fix the positions of oo", on the line G H produced, we shall have to set off the positions of ó' o" by means of the station pointer, the angle at H not being a right angle; the same is to be observed of the line HK, the observed angles on which are subtended by the line HI.
With regard to the line M N; to set out this line across the offing parallel to H K, both HM and KN being already set out at right angles to H K, and the distance H M having already been determined by the observation there taken, as HMB; the length of H K is known, or may be ascertained by any of the means we have already explained, and M N, being parallel to it, and between the same parallels, is equal to it, we have therefore only to set out the distance KN, or, which will be the same thing, determine the position of the buoy at N, by observation from H, which may here be done either with the theodolite or the sextant; that is, by taking H M as radius, and M N =H K as tangent of the angle MHN; or dividing H K by H M, which will give the natural tangent of the angle required; this operation we will not repeat, as it has already been just explained. The line M N being buoyed out, we may take any required observations along it on the two stations H M or K N. The point O is fixed by the two angles COD,DO E, and laid down on the plan by means of the station pointer; we may in the same manner determine P, Q, R, or any other number of irregular stations that may be required, from which any special observations may have been made, as for instance by the diving-bell, or by boring, With a little study the reader will be able to make many more observations on this diagram.
Another method of buoying out a harbour is illustrated at Fig. 118; the stations a, b, c, d, e, being fixed, buoys may be laid at any convenient spots, as A, B, C, and lines through them and any or all of the stations may be starred off, one after another, and the required observations may be made along the lines thus buoyed out, in the manner already explained. In case any of the above stations, as a, b, c, d, e, are inconveniently placed, as
, for instance on the top of a cliff, then the positions of others may be determined from A B, and C, lower down the cliff, where they may be secured a few feet up from the beach, as for instance two or three pieces of plank knocked together about four feet square, and painted black or white, as may best show up against
INSTRUMENTAL OBSERVATIONS OFF THE COAST. 193
a dark or light cliff; the position of these may be checked by observations made upon them from two or three stations on shore. .
With regard to the kind of boat required, much will depend on the locality, the extent of observations to be made, and their description as regards the soundings, as whether with the lead, the plunger, or other means ; this boat must be stiff, and well manned, so as to be under command, but not crowded; the observer should, if possible, register his observations himself; he will require a man to take the soundings with the lead, as there will be quite enough for the observer to do to take angles, and enter notes; besides which he must keep his hands dry; the person sounding should be trustworthy, though still carefully watched. Sometimes a boat with two good oarsmen and a man at the helm will be sufficient, for great speed is not required; and at others such a boat as a ship's launch may be required; the reader will form a better idea of what he is likely to want when we come to the subject of sounding, and observes the conditions to be fulfilled. When he has obtained sufficient practice, the observer will often be able to make his observations without mnoring, and therefore it will be necessary to get accustomed to h.. work by use, before he takes to it in earnest ; at other times it will be necessáry to anchor for every observation; this must depend on the kind of work that has to be done; it will be necessary to moor the boat when several angles have to be taken from one station ; when only two have to be taken, select stations if possible, so that the two angles shall be compassed by the limb of the sextant; after having taken the second angle, repeat back, as this should bring the vernier to zero.
The box-sextant will often be found sufficient, but we confess our predilection for the sextant, the limb being much more legible, particularly after a little use. In using the sextant, grasp it by the handle, firm but not tight, because this will make the hand shake; let the handle be held with the four fingers of the right hand, leaving the thumb to work on the barrel of the adjusting screw of the telescope when this is in use. Place the second and third fingers of the left hand steadily against the limb and its support beyond the tangent screw, the objects to be observed being supposed to be brought already nearly in contact; the finger and thumb are now free to act steadily together on the tangent screw, and without any strain. The elbows should be well raised, instead of laying near the sides, so that the body may move freely from the hips, which counteracts the motion of the boat.
During fine calm weather as much boat work as possible should be got over, which the landsman will find no small comfort, merely employing an hour or two in the evening in laying down sufficient of the work to see how it progresses, and how it lays on the plan ; a few hours thus passed in the evenings will be well repaid by keeping the mind easy during the progress of the work.
From the above observations we hope it will be obvious, why careful study should be given to placing certain stations and measured or calculated lines in such positions as to be conveniently seen from the offing and off the shore, and so also that there shall be as few very obtuse angles as possible. These lines and stations often require to be determined before any others, and then other lines and stations laid out subservient to obtaining the lengths and positions of the first. We hope also that it has been made evident how very valuable may be the most simple knowledge of the mere elements of trigonometry, and how very easily it may be acquired.
We shall now conclude this portion of our subject, by observing that it is not necessary to take an instrumental observation at every sounding, it being often sufficient to take these at every fourth or fifth sounding; the intermediate distances between the observed stations being calculated by some twenty or thirty strokes of the oars; the same number being taken from sounding to sounding, the distances between the observed stations are divided into equal spaces, according to the number of times this has been done; with care, considerable time may be often saved by this means.
With regard to the number of observed stations, much must depend on the profile of the surface at bottom, for where this is irregular or the fall rapid, a greater number of soundings will be required than where we have a regular and gentle fall.
Soundings. The object of taking soundings is twofold; to obtain the depth of water over the proposed harbour and in the offing, at ordinary low-water spring tides, and also indications as to the surface of the bottom when the lead merely is used, and at some depth under it when other means are resorted to.
The Hand-lead. The instrument most generally used for the purposes of the engineer is a chain such as that made by Messrs. Elliot, which is divided into feet by marks on which the numbers are legibly engraved, each foot consisting of inch links, so that the depths may be taken in feet and inches very easily; these chains are made in lengths of 50 and 100 feet; at one end is attached a leaden weight, varying from five to ten pounds, according to circumstances, a mean between the two answering the purpose best generally; it is of a long conical shape, from two to three inches in diameter at bottom, which is hollow and filled with tallow, to which any loose particles at the bottom get
attached when the lead strikes it; when the lead falls on rocks or stones, it will be bruised by the blow. If the lead is allowed to drag at the bottom, whilst the chain is held in hand, jerks will be felt as the weight strikes against rocks or boulders, and where these are rough and sharp, the lead will be found torn when drawn up; when these unwelcome occupants on the surface of the bottom are encountered, careful observations of their whereabouts are required that the site all round may be carefully examined to ascertain the extent of such foul ground; the jerks will be found to subside as the lead leaves the rocky surface for one of sand or clay. The height of projection of these rocks above the surface may, of course, be easily ascertained by the chain. Care, however, is required in allowing the lead thus to drag at bottom, or it may get so entangled or wedged between two rocks, that it may be impossible to free it, and the chain gets broken; a repetition of this might become expensive ; for the purpose, therefore, of dragging it is better to have one or two spare leads, one of which may be attached to a rope so protected as to prevent its getting chafed against the bottom, the chain with its weight being reserved for taking depths.
It will not fail to be observed, that this is but a very superficial way of obtaining indications as to the nature of the bottom, for a thin surface of mud, sand, or clay, may overlie a substratum of rock, the presence of which would thus remain unknown, and the whole survey, as far as the value of the holding ground, would be a fallacy; and even if the soil at the surface was of any depth, its quality as to power of holding would equally remain unindicated.
The Plunger.--In conjunction with the hand-lead, there is another instrument which is often used to ascertain the nature of the substratum to a depth of from one foot to two feet below the surface; this instrument is called the plunger, and is illustrated at Fig. 119. It consists of an iron bar, about six feet long and about 1} inch thick, armed in the middle with a weight of from thirty to one hundred pounds; above the eye end may be attached a piece of board to act as a float, and to assist the perpendicular descent of the plunger, which is pointed at one end to enter the ground; the length between the point and the weight is jagged, as shown in the drawing, so as to bring up portions of the ground pierced through by the instrument, which being suddenly let down with such a weight enters the subsoil to some depth-the deeper accordingly as the weight is greater. A heavy weight, however, has its disadvantages, inasmuch as it is clumsy to handle, and to get in and out of the boat; it also requires a larger float; moreover, when the plunger is armed with a very great weight and