Akin to the subject of _Double Lights_, is that of _Leading Lights_, the object of which is to indicate to the mariner a given line of direction by their being seen _in one line_. In most instances, this line of direction is used to point out the central part of a narrow channel; and the alternate _opening_ of the lights, on either side of their _conjunction_, serves to indicate to the mariner (who ought to conjoin with his watching of the lights the observation of the elapsed time and also frequent soundings) the proper moment for changing his tack. In some places, the line of conjunction of the lights is placed nearer to one side of a channel than the other, according as the set of the tides, or the position of shoals, may seem to require. In other situations, this line only serves as a _cross-bearing_ to shew the mariner his approach to some danger, or to indicate his having passed it, and thus to assure him of his entry on wider _sea-room_. Similar considerations to those which determine the difference of elevation for _double lights_ regulate the choice of the distance between two leading lights; but the question is less narrow, and may be generally solved graphically by simply drawing the lines on an accurate chart of the locality. In some few situations, the configuration of the coast does not admit of a separation between the lights, sufficient to cause what is called a _sharp intersection_; but, in most cases, there is room enough to place them so far apart, that but a few yards of deviation in the vessel’s course, from the exact line of the conjunction of the lights _in one_, produces a distinct opening between them on the opposite side of that line. In order to insure the requisite sharpness of intersection, the distance between the lights, wherever attainable, should be not less than _one-sixth_ of the distance between the more seaward of the two Towers and that point at which the seaman begins to use the line of conjunction as his guide. I have only to add, that in situations where the land prevents a considerable separation between leading lights, they should be placed as nearly on _one level_ as is consistent with their being seen as _vertically_ separated, so as in some measure to compensate for their horizontal nearness, by rendering their intersection more sharp and striking than it can be where the observer must draw from the upper light an imaginary perpendicular in his mind, and then estimate the separation of the lights by the sine of an angle, which decreases as the difference of their apparent elevations increases. ~Distribution of Lights on a Coast.~