When he has overpassed the first line of defence, the navigator encounters a second circle—“secondary-lights”—composed of lighthouses of the second and third orders, indicating secondary capes, reefs, and sand-banks, to which it is prudent to give a good offing. When the mouth of a river or the entrance of a port is only accessible by narrow channels, whose direction an experienced and veteran pilot can hardly determine by night, other lights of the same class are placed in the line of the channel, and point out the exact course which should be taken. Finally, when the ship has arrived near the port which is the goal of her voyage, she perceives lights—“harbour-lights”—upon its piers or breakwaters, which guide her to her much-wished-for berth. * * * * * When the best positions for illumination have been selected, the most difficult task is, or rather was, to provide for their easy distinction, so that the sailor may not be misled by too close a resemblance of one to another. Suitable variations and modifications have been, fortunately, supplied by the valuable labours of Fresnel, and of the engineers who have followed in his track. At first, however, the embarrassment was considerable. Thus, the code laid down by the celebrated French Commission in 1825, admitted of only three characters for lighthouses of the first order: the “fixed light,” the “revolving minute light,” and the “revolving half-minute light.” But it was soon discovered that merchant seamen did not sufficiently heed the differences observed between the intervals of the appearance and disappearance of the latter lights; and the number of lighthouses, moreover, having multiplied beyond all prevision, it became absolutely indispensable to allow of a greater number of distinctive characters. Now-a-days we recognize five: the “fixed light,” the “flashing light,” the “revolving,” the “intermittent,” and the “double lights in one tower.” The “flashing light” is that which shows alternately two flashes and two eclipses, or more, in the interval of a minute. The “flashing light,” the “intermittent,” and the “double lights in one tower” were all first proposed and introduced into Scotland by the late Mr. R. Stevenson. The lustre of the “revolving light” gradually increases to a maximum, and diminishes to a minimum, until wholly eclipsed, at equal intervals of half a minute, one, two, or three minutes, and sometimes thrice in a minute. We designate it an “intermittent light” when the ray suddenly appears, remains visible for a moment, and afterwards is again suddenly eclipsed for a brief interval. * * * * *