The Dhu Heartach Lighthouse. 3
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THE INSTITUTION OF CIVIL ENGINEE R S. SESSION 1875-i6.--PART IV. SECT.1.-MINUTES OF PROCEEDIKGS. April 25, 1876. GEORGE ROBERTSTEPHENSON, President, in theChair. No. 1,456.-“ The Dhu Hearta.ch Lighthouse.” By DAVIDALAN STEVENSON,B.Sc. Edin. THECommissioners of Northern Lighthouses have recently, after six seasons’ work, completed the erection of a lighthouse toweron the Dhu Heartach Rock. With many incidentsof difficulties overcome and of dangers met, it has a history very similar to the Edystone, the Bell Rock, and the Skerryvore lighthouses, which, as is well known, have been fully described in the narratives of their re- spective Engineeml But in drawing up the following notice, the Author has restricted his remarksto a bare record of engineering facts, the materials for which have been derived from the annual reports of Messrs. D. and T. Stevenson, MM. Inst. C.E., to the Commissioners, and also from personal knowledge acquired during periodical visits made while the works were in progress. Althoughthe necessity foralighthouse on Dhu Heartach had been frequently,brought before theNorthern Lighthouse Board, it was not tillithe year 18G7 that the Commissioners found themselves in a position to proceed withthe work. Thechart (Plate 1) shows the important position occupied by Dhu Heartach with regard to the entrance to the Irish Channel, to the Frith of Clyde, andto the navigation of the Minch. Thelighthouse of ~~ VZe “Narrative of the Building, andn Description of the Construction of the Edystone Lighthouse, &C.,” by J. Smeaton, London, 1871 ; “ An Account of the Bell Rock Lighthouse, &C.,” by R. Stevenson, Edinburgh, 1S21; and “ Accouut of the Skerryvore Lighthouse, &C.,” by A. Stevenson, Edinburgh, 1848. [1875-76. N.s.] B Downloaded by [ Swinburne University] on [26/09/16]. Copyright © ICE Publishing, all rights reserved. 2 MINUTES OF PROCEEDINGS. Skerryvore is 194 nautical miles to the north-west, and the Rllins of Islay 27 nautical miles to the south, leaving between the two existinglights an intermediate stretch of unlighted coast of 43 miles: DhuHeartach is 14 nautical miles from the shore station on theIsland of Earraidin Mull, thenearest land. A dangerous reef, called the Torrin Rocks, extends from the Ross of Mull for 4s nautical miles in the direction of Dhu Heartach, which, indeed, may almost be regarded as t.he outer extremity of the reef, although it is 9 miles beyond the outermost rock. To homeward- bound vessels making the shore, not having sighted either Skerry- vore or the Rhins of Islay, no warning whatever announced their approach to this treacherous group of rocks, which have proved destructive to many a ship, the only evidence of the wrecks being the drift timber thrown ashore on the neighbouring coasts of Mull and Iona. The Dhu Heartach group, according to the survey of Captain E. J. Bedford (Plate l), consists of the main rock, five detached tidc- covered hummocks, and two small sunken rocks. The main rock is an isolated mass of trap, 240 feet in length and 130 feet in breadth, with a rounded top rising to 35 feet above high water of ordinaryspring tides. It is surroundedon all sides by deep water,and, excepting between the bearings W. 4 N. to N.W., none of theoutlying rocks of the group afford the main rock any shelter. Even very slight swells from the west, after break- ing on its western face, divide and sweep round the north and south ends of the rock, as separate waves, meeting together at the eastern side, and leave no spot round its margin to which a boat can, even with a moderate westerly swell, safely approach ; while the depth of water close to the rock, and the form of the bottom to the west, admit of waves of great size and power being thrown upon it during heavy storms. An important matter,connected with theestablishment of a light in such a locality, is the selection of a suitable site on shore, as a basis for conducting the works, and for the erection of houses for the lightkeepers and crew of the attendingvessel, who are permanently in charge of the lighthouse. After acareful examination of the adjoining coast, Earraid, at the western extremity of the Island of Mull, was fixed on as the most suitable spot for the shore station. Though 14 miles distant from the rock, it was the nearest place with which communication could be kept up, and presented the advantage of affording an abundant supply of excellent granite; and there, accordingly, the ground for the permanent dwellings of the lightkeepers and seamen, as well as for quarries, workyards, Downloaded by [ Swinburne University] on [26/09/16]. Copyright © ICE Publishing, all rights reserved. THE DHU HEARTACH LIGHTHOUSE. 3 stores, &C., was acquired by the Commissioners from the proprietor, his Grace the Duke of Argyll, who afforded every facility that could be desired. There also a wharf was erected for shipping the materials, no dock or harbour works being required, as the Sound of Earraid is tolerablysheltered. As regards theform of the tower, Messrs. Stevenson, after trying various curves, fixed on the parabola as best suited to the required conditions, and the lighthouse tower is a parabolic frustum sur- mounted by a plain cavetto, abacus, and parapet, the upper course of which is 107$ feet above the foundation (Plate 2). It was con- sidered that the linesof the parabolic shaft, running withoutbreak into those of the cavetto, would produce a betterarchitectural effect, in a tower of the height proposed for Dhu Heartach, than could have been obtained by the introduction of a belt course, 80 successfully used in highertowers. Besides, on engineering grounds, it was deemed inadvisable in this particular case to oppose a belt course to the seas which might be projected up the face of the tower. The maximum diameter of the Dhu Heartach tower at the base is 36 feet, and the minimum at the top is 16 feet. The door- way is 32 feet above the foundation, and the interioris divided into six compartments, or rooms, affording accommodation amounting to 5,500 cubic feet. The total weight of the tower is 3,115 tons, of which 1,840 tonsare contained in the solid base. The excellent grey granite, of which the tower is constructed, was quarried in the immediate neighbourhood of the shore station. Having made these general statements, the Author has now to offer the following brief outline of the progress of the works during each of the six yearsoccupied in theirconstruction, noticing, at the same time,such engineering factsas may, it is believed, prove interesting. 1867. The authority t,o begin the works was only obtained on thc 11th of March, 1867, so that little more than preparation could be done during that season. In the expectation that the landings on the rock would prove peculiarly difficult and uncertain, and that water and provisions might often be got upon the rock, although not pos- sible to land workmen, it had been resolved to erect, as at the Bell Rock and at Skerryvore, a barrack for the workmen, so that the work on the rock might be prosecuted at times when the sea was too high to admit of lauding. It was also considered a source of safety in case of the workmen not being able to get off the rock from a sudden rising of the sea. The construction of this barrack, B2 Downloaded by [ Swinburne University] on [26/09/16]. Copyright © ICE Publishing, all rights reserved. 4 DlINUTES OF PROCEEDINGS. hereafter referred to, was obviously a part of the work with which no time should be lost. A powerful steam-tug was therefore chartered,and twenty-seven landings were made between the 25th of June and the 3rd of September, after which no landing could be effected. But,, short as the season’s work was, the men did good service in beginning the excavation for the foundation of the tower, and in erecting the first tier of the barrack, which was left to encounter the winter’s gales. The winter of 1867 was occupied in building thesteamer ‘‘ Dhu Heartach,” to be employed chiefly in towing the stone lighters and other service, and in constructing lighters, cranes, and plant generally, so as to be in readiness for the next season’s work. 1868. The attending steamer arrived at Earraid. on the 14th of April, to take advantage, as was anticipated, of the easterly winds, which generally prevail during the spring months. But strong westerly winds continued almost without intermission till the end of June, during which period of two and a half months no work was done upon the rock. Various attempts were from time to time made to land, but withoutsuccess, except on the 4th of May, and even then thc men could only remain one hour and a half; but, time was afforded to ascertain that the work of the last season had stood on the whole well, considering its unfinished state. One section of the iron ring connecting the heads of the uprights of the first tier of the barrack, at a height of30 feet above high-water level of spring tides, had, however, been carried away. Again on the 18th of May six men landed, it being unsafe to land a greater number, but the sea continuing to rise, they were taken off after being about one hour and a half on the rock. The weat,her then became more unfavourable, and no landing was made tillthe 29th of June. The wind, indeed, never fairly settled into the east,so that more or less of a westerly swell continued throughout the whole season, being the most unfavourable weather for the progress of the work.