The matter of keeping track of time puzzled Robinson very much. It was getting more difficult every day to keep it in his memory. He must write down the days as they slip by, but where and how? He had neither pen, ink, nor paper. Should he mark every day with a colored stone on the smooth side of the huge rock wall within whose clefts he had dug out his cave? But the rain would wash off the record and then he would lose all his bearings. Then he thought of the beach, but there the wind and waves would soon also erase it.

He thought a long time. "I must find something," he said to himself on which to keep a record. "I must also know when Sunday is. I must rest one day in the week. Yes, I must find something," he said, "on which to write." And finally he found it. He chose two trees standing near each other and then sought for a small sharp stone, which he could make still sharper by striking it on another. When he had got this pen ready he cut into the bark of one tree:

Shipwreck, Sunday, 10th of September, 1875.

He made seven cuts in a row for the seven days in the week. The first cut was longer than the others. This was to represent the Sunday. At sundown every day he made a new cut in the bark.

The other tree he called the month tree. On its stem he was to cut a mark every time his week tree told him a month had passed. But he must be careful, for the months were not of equal length. But he remembered that his teacher had once said in school that the months could be counted on the knuckles and hollows of the hand, in such a way that the long and short months could be found easily and he could tell in this way the number of days in each.

Robinson worked at enlarging his shelter a little every day. He was sorely at loss to find something in which to carry the dirt away from the entrance, or enough so that it would not choke up the opening. A large clam shell was all he could think of at present. He would carry the dirt to the entrance and some distance away, and then throw it. Fortunately the ground sloped away rapidly, so that he needed a kind of platform before his door.

He was careful to open the cleft at some distance above the large opening. For the air was damp and impure in the shelter. But with the opening made high above, fresh air was constantly passing into, and impure air out of, his cave. Light, too, was admitted in this way.