Page:Encyclopædia Britannica, Ninth Edition, v. 11.djvu/480

458 HARBOURS powers of nature that are subject to no calculation, I trust it will be deemed prudent not to omit, in such a case, any thing that can without difficulty be applied, and that would be likely to add to the security." This statement of our greatest marine engineer indicates the propriety of carefully collecting any facts that may help us to a more accurate estimation of those forces which he regarded as being " sub ject to no calculation." At the Bishop Rock lighthouse, a bell was broken from its attach ments at the level of 100 feet above the high water mark during a fale in the winter of 1860, and at Unst, the most northern of the hetland Islands, a door was broken open at a height of 195 feet above the sea. To these facts it may be added that, as proved by the testimony of an eye-witness, a block of 50 tons was moved by the sea at Ban-ahead, one of the Hebrides. But still more extra ordinary effects have been observed at Whalsey in Shetland, where heavy blocks have been quarried, or broken out of their beds in situ, on the top of the Bound Skerry at a great elevation above the sea. On the south-east side, about 870 feet from the low water mark, and at a height of 62^ feet above its level, there occurs a remarkable beach of angular blocks varying in size from 9J tons downwards, which are huddled together just as one would have expected to find, had they been elevated only a few feet above the high water level. Towards the north-east at the Iffvel of 72 feet above the sea, in ad dition to many smaller blocks which had evidently been recently detached, there was one 5J tons in weight. It presented the appearance of recent detachment, having a fresh unweathered look. Within 20 feet of the spot where it lay, there was a comparatively recently formed void in the rock, which upon examination and com parison by measurement was found to suit exactly the detached block. Here then was a phenomenon so remarkable as almost to stagger belief- -a mass of 5 tons not only moved but actually quarried from its position in situ at a level of 72 feet above high water spring tides ; and higher up still there was another detached rock weighing no less than 13J tons tilted up in a peculiar position, and underneath which numerous angular masses had been wedged, obviously by aqueous action. But by far the most striking effects as yet recorded were at the harbour works of Wick, which were commenced in 1863, and con sisted of blocks of from 5 to 10 tons set on edge, first built above high water neap tides with hydraulic lime, then with Roman and latterly with Portland cement. In October 1864, 300 feet of the contractor s staging were carried away ; and greenheart were afterwards substi tuted forMemel piles, but these too were broken by the sea. The first portion of the wall was, in conformity with usual practice, founded at 12 feet below low water, but 18 feet was subsequently adopted, and the rubble has since been washed down to 15 feet below that- level. In 1872 a huge monolithic block of concrete, weighing in all 1350 tons, was removed en masse out of its position and carried to leeward of the breakwater. Extraordinary as this may appear, it was surpassed in 1873, when another concrete mass which had been sub stituted for the one that was moved, was in like manner carried away, though it contained 1500 cubic yards of cement rubble, the weight of which was about 2600 tons. Dec. 10 5 1 2 I 14- 4 C J I 3 9 10 11 19 Inches. I- l_ | j. | 1 nas The marine dynamometer was constructed by Mr T. Stevenson an- for the purpose of ascertaining the force of the waves against an ob- i stacle. DEFD (fig. 6) is a cast-iron cylinder, which is firmly bolted ne at the projecting flanges G to the rock where the experiments are to be i- made. This cylinder has a circular flange at D. L is a door which is icter. opened when the observation is to be read off. A is a circular disk on which the waves impinge. Fastened to the disc arc four guide rods B, which pass through a circular plate C (which is screwed down to the flange D), and also through holes in the bottom of the cylinder EF. Within the cylinder there is attached to the plate a very strong steel spring, to the other or free end of which is fastened the small circular plate K, which again is secured to the guide rods B. There are also rings of leather T, which slide on the guide rods, and serve as indices for registering how far the rods have been pushed through the holes in the bottom EF, or, in other words, how far the spring has been drawn out by the action of the waves against the disk A. These disks were from 6 to 9 inches in diameter. The greatest result obtained by the dynamometer at Skerryvore Lighthouse, in the Atlantic, was during the heavy westerly gale of 29th March 1845, when there was registered a force equivalent to 6083 ft, or nearly 3 tons, per square foot, deduced from the area ex posed. The next highest was 5323 ft. In the German Ocean the greatest result obtained at the Bell Rock was 3013 ft per square foot. But subsequent and much more extended observations at Dunbar, in the county of East Lothian, gave 3 tons ; while at the harbour works of Buckie, on the coast of Banffshire, the highest result of observations, extending over a period of several years, was 3 tons per square foot. Mr David Stevenson, when referring, in 1835, to the lakes in North America, pointed out that the general depth of water must, in order to allow the waves to be fully formed, be considerable, and that the sea must be unob structed by shoals, and also of sufficient area to admit of the wind acting fully on the water. If in front of a harbour shoal water extends seawards for a considerable distance so as to form an extensive flat fore shore, the depth of water above it becomes the true limit of the maximum wave, whatever may be the general depth of the sea outside. But at Arbroath harbour and at Alne mouth, the heaviest waves are tripped up by the shoal water near the mouths of the harbours, and are not found so dangerous to the works inside as the smaller waves which reach the beach in an unbroken state. Mr Scott Russell states that waves break when they come into water of the same depth as their height. At Wick, however, the height of the waves above the mean level was about two-thirds of their height. In 1870, at Scarborough, it was found that waves of from 5 to C| feet broke when the depth was 10 feet 3 inches below their troughs, and 6 feet waves broke in water 13 feet 8 inches deep, which would for waves of this class give h = ^d, where d is depth below mean level, and h the height from hollow to crest. The late Dr Macquorn Eankine has shown that, theoreti cally, the mean water level is not situated half way between the crest and trough of the sea. The following formulas furnished by him give the mean level of the sea from the height and length of the wave. These formulae are exact only for water of considerable depth as compared with the length of the wave. For shallower water they are only approximate : Let L be the length of a wave, H the height from trough to crest; then, diameter of rolling circle = - , radius of orbit of O * J. 41 1) particle = H, and elevation of middle level of wave above still 3-1416H 2 -or.H 2 water - = v854 . 4L JL Consequently TT TT 2 Crest above still water = - + 7854^- ; 2 L Trough below still water -? - "7854^. 2 Lt Deep-Water Harbours. Harbours of refuge are distinguished from tidal harbours mainly by the superior depth of water which they possess and the larger area which they enclose. The requisites are shelter during storms, good holding ground, and easy access for shipping at any time of tide and in all states of the weather A breakwater, though a passive, is yet a real agent, having true work to do. During storms many thousand tons of water are elevated and maintained above Depth in front of a har bour regulates the height of waves. Depth in which waves

break.