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.
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.
A few days' practice is not thrown away if the canoist seizes every opportunity of performing under easy circumstances feats which may at other times have to be done under necessity, and which would not be so well done if attempted then for the first time.
Let him, therefore, as soon as possible, become adept in crossing above or below a single sunk rock with his boat's bow pointed to any angle of the semicircle before him.
Next we have to consider the cases in which more than one rock will have to be avoided. Now, however great the number of the rocks may be, they can be divided into sets of three, and in each of the figures 3, 4, 5, 6 it is supposed that (for reasons which may be different in each case, but always sufficient) the canoe has to pass between rocks A and B, and then between B and C, but must not pass otherwise between A and C.
In fig. 3 the course is below B, and above C, being a combination of the instance in fig. 2 with that in fig. 1.