A Treatise On The Principles And Practice Of Harbour Engineering

SURVEYING, MARINE AND SUBMARINE. 49 fully efficient for the purposes stated above. It would, however, take too long to enter into these, and we must dismiss the subject at this point in order to return to the main theme under considération. In the example (fig. 44) of a line of soundings taken and plotted by Sutcliffe’s Apparatus, the letters A, B, C, D represent the position of cross sights, by means of which the operator verifies his distances. During the process of taking soundings in a tideway, two points demand the constant attention of the operator. One is the alignment of the dips, and the other is the mutation of the water level. Alignment is difficult to maintain in a cross current, and the boat needs to be carefully watched to see that it does not drift out of its course. If a line of soundings be taken from the shore or quay out towards the open, a couple of poles or other suitable uprights, one at the water’s edge and the other some 30 or 35 feet back, should afford adéquate guidance. When the bank is steep, it may be necessary to give the rear pole greater elevation than the other in order to be able to range them both with the eye from a lower level. If a line of soundings lie between two fixed points, such as stakes fixed into opposite banks of a river, the distance between them not being very great, a rope may be stretched taut from one to the other. In this case, by providing the rope with tags at regular intervals, the exact distance of each sounding can be recorded. When there is only one fixed point, distances may be read off a cord or rope, similarly tagged, and paid out from the boat as it proceeds. Failing this, the position of the boat at each dip must be fixed by angular measurement from the shore, with the aid of the sextant or théodolite. Variations in the Water Level should be noted at regular and stated intervals (say every ten or fifteen minutes) by an observer stationed at a tide- gauge adjacent to the site of operations. The operator in the boat also notes the dips which correspond to the same in- tervals of time, and, by subséquent com- parison with the tide-gauge readings, the proper correction can be made by which all the soundings are referred to one datum line, either local and temporary, or established and general. A Tide-gauge is an appliance for the purpose of indicating changes in the sea surface level. In its simplest form it consists of an upright stake or post (fig. 45) driven into the shore or bank, and graduated to linear measure. In some situations a single post may suffice to indicate the whole tidal range; in Stones Stones Fia. 45. —Temporary Tide-gauge on Beacli. other cases a number of posts may be necessary, extending across a sloping shore from high water level to low water level and forming a series of steps. 4