strikes upon rocks, it sometimes gets bent and even broken. It is therefore well to have two in the boat, armed with two different sized weights; the lighter one being used first, if there is no indication of rock and we wish to test the quality of the ground to a greater depth, we then let down the plunger armed with the heavier weight. This instrument has been in use for very many years; it will not, however, indicate the nature of the bottom beyond a certain depth; neither will it discern between the natural rock and a mere deposit of stones imbedded in the surface; a little patience and a few trials will, however, sometimes assist us in this, but not constantly. The anchor should always be examined when drawn up, the fluke often bearing good evidence. In very many cases the hand-lead and the plunger together will afford very satisfactory proof as to the nature of the bottom, more particularly where the shore gives no indication of rock, or where the geological formation does not give reason to apprehend anything of the kind; where the water is not of too great depth, the ground may be probed by means of a long pole armed with an iron bar. Diving. The next method of ascertaining the nature of the holding-ground is by the reports of a trustworthy diver walking over a portion of the ground. He is provided with an iron bar four or five feet long, attached to a buoy with a two or three-inch rope; he drives this bar into the ground with a mawl, or presses it in, and may be led some distance round the boat; if provided with a rope some twenty or thirty feet long, he may examine the ground some distance round his ladder, and even further by sight, which, however, will depend on the state of the water; he can also send up samples of the ground, and to test his accuracy he may be led to and from a spot examined, and back to it again, and his reports may be compared. The bar when hauled up, after being sunk in any adhesive ground, will bring up some of the earth sticking to it; and if it has been driven in to any depth will often require a windlass, and several men to haul it up. Although the diver may sling samples which may then be drawn up, his reports must be taken as he gives them to a very great extent; it is therefore indispensable that he be trustworthy, and also very desirable that he should possess sufficient nautical experience to understand the subject on which he is reporting. The points where these observations are made will, of course, have to be determined by carefully measured angles; or they must be fixed by the intersections of lines on shore, these lines being on stations, buildings, or other objects on the plan. The spots thus examined by the diver should be shown by circles HOLDING-GROUND.-ANCHORING. 197 equal in diameter to the distance actually examined; and these circles should be numbered in accordance with the numbers in the note-book. To Boring. The next method of testing the bottom of a harbour is by boring down through a length of iron-piping, about three inches in diameter, and in lengths of about six feet, which is done at about the time of low-water, from a platform rigged over two boats. Under any circumstances this is a troublesome operation in deep water, where there is any swell or motion; and although it will be much less difficult to sink into clay or sand, it is very different on striking a boulder or rock, where every inch downwards has to be gained by "jumping.' pierce through a large boulder in deep water will be a rather serious undertaking, or into the natural rock; and the boringtool will not ascertain whether the work is being carried on a natural bed, or an accidental deposit, any more than the plunger will; but where it is only desirable to ascertain the depth of a stratum into which the auger will cut with average despatch, then boring will generally be the best and readiest means to ascertain the fact as to depth of sand, clay, mud, &c. &c. Of the above four means, viz., the hand-lead, the plunger, the diver, and the boring-rod, any one, or all conjointly, may be used according to circumstances, and as one of these may point to the necessity of using another; the nature of the bottom of the harbour as holding-ground is a condition about which there must be no doubt, whatever the means, or the number of means, we may have to resort to before we can come to a conclusion satisfactory to ourselves or to others. Anchoring-In all the above operations the engineer will be more or less assisted by nautical men, and with regard to testing the quality of a holding-ground by anchoring, this assistance will become all the more indispensable; the value of this test consists in leading at once to a practical conclusion as to the actual quality of the holding-ground. The various means adopted to prove this by anchoring are very numerous; employing a steamer, she may be anchored in six or seven fathoms at low-water, veering out the length of cable that may be thought sufficient to hold in a gale of wind; this may be a whole cable, and afterwards less and less; the steamer may then be backed by the paddles, and as soon as the cable indicates the strain being felt, the lead may be dropped over, and it may then be observed whether the anchor starts under the power of the engines, a careful reckoning being *The reader who has had no experience with boring tools, will find a full description of them in Railway Construction, by the author of the present volume. kept of the number of revolutions made at the time; times of slack and smooth water should be sought, when convenient, for making these experiments, and consideration must be given to the condition as to whether the harbour is to be a close harbour, the amount of shelter, the part occupied by the vessel as regards prevailing winds, &c. According to the tonnage of the vessel, the weight of the anchor used is five cwt. to one ton; if in such a depth of water as that above mentioned, the anchorage is tried by letting out only some twenty-five fathoms, conditions may arise such that the anchor has no chance of getting hold of the ground, whereas were twice such quantity veered out, the anchor would get a fast hold. Whilst to let out too little cable may be one extreme, the other may be easily run into by "making arrangements" such as could not be worked out under the natural state of things in which a vessel would make her way into harbour. It is also to be remembered, that although a sailing vessel, by doing all that may be done to check her way, may assimilate to the conditions of a steamer, still it is not possible to obtain as ready a command over her sails as the engineer has over his engine; it is therefore necessary in the log to keep a correct account of all that is done in this way to try the quality of the holding ground, for should the tide set rapidly across the mouth of a harbour, a good way must be kept upon a vessel to enable her to get in; the quarter of the prevailing gales will also have to be considered. Reducing Soundings.-During the greater part of the time that soundings are being taken, to ascertain the depth of water the tide is rising or falling, and therefore to reduce the depth to one standard or level, it is necessary to have a correct record by time of such rise or fall; it has already been observed that this level is generally "Low Water Ordinary Spring Tides." It is therefore necessary that the observer in the boat should keep a correct time as to his soundings, and that a most trustworthy person be employed to make the observations also by time of the rise and fall of the tide as indicated by a tide gauge established for the purpose. By this means the observer has on returning to land the opportunity of obtaining the rise and fall at stated times, and therefore of reducing his soundings to the standard level. At Fig. 120, which is a slight sketch of Holyhead harbour, the manner in which the depths of soundings are entered on the plan has been shown; the dotted lines, however, are only laid down on the working or draft plan. With regard to the nature of surface at bottom of harbour, this is shown by patches or circles, or by long bands, accordingly as observations have been TIDAL OBSERVATIONS.-THE TIDES. 199 made in various spots, or in a continuous route; and these various marks are coloured by conventional colours, an explanation of which is carefully entered on the plan; the patches or circles are all numbered in Roman figures according to the notes previously made, and to distinguish them from the depths of water. If the nature of the bottom has only been ascertained by the lead or plunger in a continuous line, it is shown merely a strong dotted line; if by a diver, then the width tried by his probing bar is laid down on the plan, according to the widths of surface examined. Any other system may be adopted, provided it expresses distinctly the nature of the information obtained; thus small geometrical figures, as circles, squares, triangles, crosses, &c., may be used to distinguish the different features of the surface of the bottom, provided that their meaning be entered on the plan. Similar or analogous marks and signs may be used for expressing the power of the holding-ground, according to the number of revolutions made by the engine before the anchor was moved; two parallel lines drawn quite close together are generally the means of showing the length taken by steam-vessels to bring up, and along these lines is shown in "knots" the rates at which the vessels were going when the anchor was let go. The quantity of information to be thus shown is often so great, that to enter it all on one plan would result in a mass of confusion; two or three plans are, therefore, often prepared, on each of which is entered only one description of information; all this must in a great measure depend on the quantity and different natures of the information that has to be expressed on the plans prepared. With regard to the tidal observations that have to be made in connexion with harbour and marine surveys, they may be very simple, and at times very complex, particularly if extending over any length of time, as is generally required to be the case, or if made along any long line of coast, or up an estuary or tidal river. On the open coast there is a greater regularity in the rise and fall of the tide than in most other situations, particularly narrow channels, estuaries, and tidal rivers. In these cases the anomalies in the tides are so numerous that the tidal lines are by no means parallel during the whole period of flood and ebb, as a very little consideration will show. For when the tidal wave enters a channel, it does so with a certain velocity and volume of water; if a contraction occurs in the width of the channel, either the depth of the water or its velocity, or both, must increase according to the momentum due, so that in some situations the height of the tide may be considerably in excess of the elevation in the open sea. The greater the rise of the tide, the more irregular the longitudinal and transverse sections of the bed, the more numerous the sharp turns, bends, and contractions, the more rapid the currents, the more numerous should be the stations at which tide gauges are established. Although we suppose the reader more or less acquainted with the nature of the tidal phenomenon, it will be necessary for many younger students that a few general observations be made on the subject in such an elementary work as this. These, however, we will endeavour to give in as few words as possible. The tidal wave is due to the force of attraction exerted by the sun and moon on the waters on the surface of the earth. The curved surface of the water due to the progress of the tides from that part of the ocean where they are first produced, and which passes from one low water over the summit of the tide down into the next low water, has been denominated the tidal wave; it is so long that it takes 12 hours to move its own length, but is, nevertheless, a true wave. The force of attraction from bodies at equal distances is proportional to their masses. The attractive forces of bodies equal in mass, but at different distances, vary inversely as the squares of the distances. The effect of the combined attraction of the sun and the moon upon any point of the earth's surface, would be to draw up the waters in a heap at that point under the sun and moon, if the earth stood still long enough; but on account of the velocity with which the earth rotates, there can only be a tendency to this result. Taking the mass of the sun at 28 millions that of the moon, and its distance from the earth 400 times that of the moon, its 28,000,000 power of attraction will be 160,000 moon. = 175 times that of the This is on the mass of the earth, and not at different parts of the surface. Taking the distance of the moon from the centre of the earth at 60 times the radius of the earth, then the side nearest the moon will be at a distance of 59 times the radius, and the farthest at 61 times the radius; and the difference in the force of attraction at the nearest point and the centre of the earth, and at this centre and the farthest point will be 1 30 In the same manner, taking the nearest point of the earth to the sun at 23,999 times the radius, the centre as 24,000 times, and the farthest point at 24,001, it may be shown that the difference of attraction at the surface and at the centre is only |