Here is the line we would follow if we could go straight from Camp Jackson to Pensacola.

If we could follow it, I would only have to guess how many miles we march each day, and mark it down on the map.

But we can't go straight, because of swamps and creeks and canebrakes, so I must keep looking at my compass to find out what direction we do go; then I mark on the map the route we have followed each day, and the distance, and each night's camp gives me a new starting point." "Yes, but Sam," said Tom, suddenly thinking of something.
"Well, what is it, Tom ?" "Suppose you guess wrong as to the distance travelled each day ?" "Well, suppose I do; I can't miss it very far." "No, but it gives you a wrong starting-point for the next day, and two or three mistakes would throw you clear out." "Yes, but I make corrections constantly.

You see, I have changed the place of last night's camp a little on the map." "How do you make corrections ?" "By the creeks and rivers.

Here, for instance, is a creek that we ought to cross about ten miles ahead.