A Treatise On The Principles And Practice Of Harbour Engineering

PIERHEADS, QUAYS, AND LANDING-STAGES. 189 superstructure. The waves will act upon the upper surface of such a ledge with disastrous effects, both to that and to the recessed pierhead of which it is a part. The latter tends to become undermined in the manner illustrated by the pierhead at Pillau.1 The best form of pierhead is that which presents an upright front to the sea on all sides, without benching or recesses of any description. By making a pierhead perfectly self-contained, the problem of connecting it by any system of bond with the breakwater proper does not arise. On several grounds, and especially in reference to breakwaters liable to considér- able settlement, it is desirable that pierheads should not be involved in any movement of the adjoining parts. Therefore, while the breakwater and its pierhead may be and commonly are in close contiguity, there should be a vertical joint between them, rendering them independent of each other’s action. (2) As a means of guidance and direction for vessels, it is desirable that a pierhead should possess some prominently outstanding feature. This de- sideratum may be met by a lighthouse (fig. 230, page 264), or mast (fig. 160), or, where these are not required for signalling purposes, by an elevated platform. A lig’hthouse, if in masonry or concrete, should be stoutly and substanti- ally constructed. Concrete work lends itself admirably to liomogeneity, there being, in this case, no difficulties to encounter such as attend subaqueous work. Masonry is more costly, in that bedding and jointing demand the most careful execution; and in many cases there must be introduced elaborate bonding courses containing dovetails and keystones, all entailing considérable expense. Of the forms adopted in connection with either of the systems, the circular is, on the whole, the best, offering least obstruction to, and assisting in, the deflection of breaking seas. A flat or plane face, however, is convenient for window, door, and other openings. The lantern being situated at the sunimit, the lower part of the tower should be utilised so as to afford a storeroom and also a shelter chamber for those whose duties necessitate their presence during times of storm. Stress of weather may, in faet, interrupt communication with the shore for several hours, if not whole days, at a time. A spiral staircase in the interior of the tower generally leads to the lantern. Pierhead lighthouses may also be constructed of steel, either in the form of open framing or with plate sheeting, and also of a cluster of cast-iron columns connected by ties and bracing. Variations in design are, in fact, numerous and almost illimitable. The lantern (fig. 242, page 272) is a glazed chamber, having a pedestal of cast- or wrought-iron, and a framing of gun metal or steel. Glazing is now generally circular rather than polygonal, as was formerly the practice. The glass is polished plate, in most cases | inch thick. Spare panes should be kept handy to replace breakages, which are likely to arise from birds flying against the lantern as much as from the effects of storms. 1 See p. 191