4150 HARBOURS Table of principal Projiortions of remarkable Breakwaters. Name Kind of Work. General Slope of Outer Face. Inner Slopes. Level of top of loose Rubble below Low Water. Level of founda tions of Wall below Low Water. From Bottom to near Low Water. Near Low Water. l"p to Hitfh Water. Above High Water. Above Hiffh Waur. Below Low Water. Top above HiKh Water. Plymouth . . . Sloping Breakwaters. Pitched slopes above high water, loose rubble below. Slopes of loose rubble. Pitched slopes of rubble. Slopes of loose rubble. Composite Breakwaters. Slopes of loose rubble, with plumb wall above high water. Do. do. Do. do. Do. do. Do. do. Vertical Breakwaters. Solid masonry. Concrete blocks below low water, and solid above. 1 to 1 If to 1 If to 1 2 to 1
to 1
2 to 1 2 to 1 14 to 1 & 6 to 1 1 to 1 & 7 to 1 [This is from bottom to 15 feet below low water.] i to 1 i to 1 4 to 1 6 to ] 5 to 1 5 to 1 5 to ] 7 to 1 5 to ] 3i to 1 wall J tol i to 1 i tol 5 to ] 3 to 1 5 to ] 12 to 1 4i to 1 7 tol wall
to 1
wall I to 1 1 1 to 1 a toi 5 to 1 1 1 to 1 5 to 1 4 to 1, & plumb wall. i to 1, & plumb. wall
- tol
i to 1. & cavetto s to 1 2 to 1 H to 1 I tol plumb wall. 7i to 1, & | to 1 wall itol | tol | to 1 |tol 2 to 1 1 to 1 4 to 1 1| to 1 1 to 1 1 to 1 H to i 1 to 1 i to i .1 to 1 i tol Feet. 3 16 15 25 ia 25 12} 6&21 23 11
15
12 18 45 20 Portrush Kingstown llolyhead Portland Cherbourg Alderney Cette Pulteneytown Dover Aberdeen price per lineal foot, and cZ = the depth in feet at high water, then P x =" The results calculated in this manner are arranged in order of their costs in the following table, from which the plumb pier at Dover appears to be by far the most costly: Name of Harbour. Depth in Feet at High Water. p <i Portland . . (32 1 90 Joliette 35 2-06 Algiers 42 3 00 Marseilles (new) 35 3-10 Plymouth 58 3 45 Holyhead 36 4 40 Alderney 37 4-60 Dover 38 9-47 ail- Miriard allows for large merchant vessels in harbours of c refuge one cable-length, which would give about four ves- ) sels per acre. Captain Calver allows three vessels per ivs of acre f r a small sheltered harbour of refuge. At Cardiff age Flats there were at one time 224 vessels anchored as close 1 f to each other as they could well be, in an open roadstead,
- " and occupying a space of 5GO acres, which would give 4
vessel per acre. Tidal Harbours. We have hitherto been considering outer breakwaters erected in deep water, and which are constantly exposed to the waves ; we now turn to piers and sea-walls which are placed within the range of the breaking surf, and which are exposed to its force for a limited period only, being some times left nearly or altogether dry by the receding tide. In dealing with waves which are admitted by all to exert a true percussive force, the question arises as to how this force may be best resisted whether by opposing to it dead weight, or a comparatively light structure, the stability of which is dependent on strong fixtures connecting it with the bottom. On this subject the late Mr Alan Stevenson made the following remarks, in relation to lighthouse towers, but his views are equally applicable to piers and breakwaters : " A primary inquiry in regard to towers in an exposed situation is the question whether their stability should depend upon their strength or their u ciyht, or, in other words, on their cohesion or their inertia ? In preferring weight to strength we more closely follow the course pointed out by the analogy of nature, and this must not be regarded as a mere notional advantage, for the more close the analogy between nature and our works the less difficulty we shall experience in passing from nature to art, and the more directly will our observations on natural phenomena bear upon the artificial project, If, for example, we make a series of observations on the force of the sea as exerted on masses of rock, and endeavour to draw from these observations some conclusions as to the amount and direction of that force as exhibited by the masses of rock which resist it successfully, and the form which these masses assume, we shall pass naturally to the determination of the mass and form of a building which maybe capable of opposing similar forces, as we con clude with some reason that the mass and form of the natural rock are exponents of the amount and direction of the forces they have so long continued to resist. It will readily be perceived that we are in a very different and less advantageous position, when we attempt, from such observations of natural phenomena in which weight is solely concerned, to deduce the strength of an artificial fabric. Another very obvious reason why we should prefer mass and weight to strength as a source of stability is that the e licet of mere inertia is constant and unchangeable in its nature, while the strength which results even from the most judiciously disposed and well-executed fixtures of a comparatively light fabric is constantly subject to be impaired by the loosening of such fixtures, occasioned by the almost incessant tremor to which structures of this kind must be subject from the beating of the waves. Mass therefore seems to be a source of stability, the effect of which is at once apprehended by the mind as more in harmony with the conservative principles of nature, and unquestionably less liable to be deteriorated, than the strength which depends upon the careful proportion and adjustment of parts." 1 It is a remarkable fact that during a summer gale 14 blocks, each 2 tons in weight, which had been permanently fixed in the Dim Heartach lighthouse tower by joggles and cement, at the level of 35^ feet above the sea, were torn out and swept off the rock, while the thin panes of crown glass in the lantern of Winstanley sEddystone tower, which were only about 5 feet higher, resisted the waves for a whole winter. The impact on marine masonry seems 1 Account of the Skerryvore Lighthouse, by Alan Stevenson, LL. B., F.R.S.E., Edin., 1848, p. 40. Effect o configu ration c rocks and of bottom of the sea on marine struc
tures.
