To cross Park-lane in the afternoon requires the very same sort of calculation as the passage across the stream in a rapid on the Rhine. The importance of this subject of "boating instinct" will be considered sufficient to justify these remarks when the canoist has by much practice at last attained to that desirable proficiency which enables him to steer without thinking about it, and therefore to enjoy the conversation of other people on the bank or the scenery, while he is rapidly speeding through rocks, eddies, and currents. We may divide the rocks thus encountered in fast water into two classes--(1) Those that are _sunk_, so that the boat can float over them, and which do not deflect the direction of the surface current. (2) Those that are _breakers_, and so deflect the current, and do not allow the boat to float over them. The currents may be divided into--(1) Those that are equable in force, and in the same direction through the course to be steered. (2) Those that alter their direction in a part of that course. In the problems before the canoist will be found the combinations of every degree and variety of these rocks and currents, but the actual circumstances he has to deal with at any specified moment may--it is believed--be generally ranged under one or other of the six cases depicted in the accompanying woodcut. [Illustration: FIG. 1.] [Illustration: FIG. 2.] [Illustration: FIG. 3.] [Illustration: FIG. 4.] [Illustration: FIG. 5.] [Illustration: FIG. 6.] In each of the figures in the diagram the current is supposed to run towards the top of the page, and the general course of the canoe is supposed to be with the current. The particular direction of the current is indicated by the dotted lines. The rocks when shaded are supposed to be _sunk_, and when not shaded they are _breakers_. Thus the current is uniform in figs. 1, 2, 3; and it is otherwise in figs. 4, 5, 6. The rocks are all sunk in figs. 1, 2, 3, and 5; whereas in figs. 4 and 6 there are breakers. The black line in these figures, and in all the others, shows the proper course of the centre of the boat, and it is well to habituate oneself to make the course such as that this line shall never be nearer to the rock than one-half of the boat's length. The simplest case that can occur is when the canoe is merely floating without "way" through a current, and the current bears it near a rock. If this be a breaker, the current, being deflected, will generally carry the boat to one side. The steering in such cases is so easy, and its frequent occurrence gives so much practice, that no more need be said about it. But if the rock be a sunk rock, and if it be not quite plain from the appearance of the water that there is depth enough over the rock to float the boat, then it is necessary to pass either above the rock, as in fig. 1, or below it, as in fig. 2.