If we go toward the gum, keeping it always in a line with the sycamore, we shall go perfectly straight, of course; and by choosing another tree away beyond the sycamore and in line with it, just before we get to the gum tree, we shall still go on in a perfectly straight line.

We might keep that up for any distance, and travel in as straight a line as a compass can mark.

Now if this country was an open one with no bogs to go around, and nothing to keep us from going straight ahead, I shouldn't need any assistance, but could go on in a straight line all day long.

As it is, I must establish a long straight line, reaching as far ahead as possible, and then pick out two things in the line, one near me and one at the far end, which we can recognize again from any point.

Then we'll go on by the best route we can till we come to the furthest object, and then I'll show you how to get the line again.
What I want you to do is to notice the 'object trees' as we'll call them, so that we can be sure of them at any time.