And, finally, Mr. Stevenson recommends different applications of sound, so as to produce distinct and powerfully audible warnings during the prevalence of a thick fog or mist:— The propagation of sound during fogs through pipes communicating with the shore,[43] or the origination of sound at the beacon or buoy itself, by condensing the column of air, or by acting on a column of water contained in the pipe. [43] In one of the Paris water-pipes, 3120 feet long, M. Biot was able to keep up a conversation, in a very low tone, with a person at the other end. Bells rung by electricity. Mr. Wilde, of Manchester, states that bells twelve or eighteen inches in diameter, placed on different beacons, and as far off as ten miles from the shore, could be tolled a hundred times a-minute by means of a three and a half or four inch electro-magnetic machine worked by an engine of about two-horse power. And, finally, bells may be rung by the simple pressure of the waves through the agency of a float, which would sink or rise according as the tide sunk or rose. This was proposed for the beacon at the Carr Rock by the late Mr. Stevenson in 1810. By the adoption of one or other of these suggestions, according to the conditions of the locality, there can be no doubt that the subsidiary illumination of our shores and their contiguous waters would be very considerably improved. [Illustration] CHAPTER IV. THE SKERRYVORE LIGHTHOUSE. The full details which we have given of the erection of the Bell Rock Lighthouse will render unnecessary any elaborate account of the mode of construction of later edifices. There are some, however, which we are unwilling to pass over without at least a cursory notice, owing either to their romantic position or to their special interest as examples of engineering skill. One of the most important of these is the Lighthouse of the Skerryvore, situated on a reef which, in all leading features, is a counterpart of the Bell Rock. It is placed in the same parallel of latitude, and occupies the identical position on the west coast of Scotland which the latter occupies on the eastern. Nor was it of old less fatal or less ominous to the mariner, but annually exacted its tribute of precious lives and wrecked vessels. A few minutes sufficed for the total loss of any unfortunate ship which dashed against the gneiss crags of the Skerryvore, and its rent and shattered timbers were quickly carried by the tide to the fishermen of the island of Tyree. Not that this formidable memorial of past volcanic convulsions was totally submerged—some of its higher points rose above the level of the highest tides: but the extent of its foundations was considerable; and even in the summer season latent dangers beset the difficult channel between its eastern extremity and the island of Tyree, which lies about eleven miles distant.[44] [44] Alan Stevenson, “Account of the Skerryvore Lighthouse” (ed. 1848), p. 41.