WE have already observed that our board was cracked at the end. We can understand this if we consider what happens to timber after it is cut down. While the tree is growing its pores are full of sap, which is mostly water. After the tree is cut, the sap begins to evaporate, and the wood shrinks. You will have no difficulty in finding, all around you, proofs of this shrinking. Flooring-boards, panels of doors, bottoms of drawers, which fit well when first put in place, all leave openings after a while by shrinking. Here are several "dowels," which were all cut from the same stick, and yesterday they all fitted well in the corresponding holes; but half of them have been soaked in water over night, and now they will not go into the holes at all. The shrinking of timber, you will find, takes place only in the width, not in the length. Examine the floor, and you will find that it is only the joints between the edges of the boards that have opened. When two boards have been put together end to end, the joint is as close as in the beginning. This fact is very striking, and should be remembered. The shrinking of wood causes endless trouble in carpentry, cabinet-work, and building, and it cannot be entirely prevented; but, by taking advantage of the fact just mentioned, it can often be prevented from doing mischief. We shall study some of these methods in Lesson 21.

When the drying of timber goes on at all parts with equal rapidity, the piece shrinks equally in all parts, and keeps its shape; but in large pieces the drying goes on more rapidly on the outside than on the inside, and this causes important changes in the shape and condition of the wood. We shall look at these changes in detail by and by, but for the present it will be sufficient to note the following facts.

First, as the outside shrinks faster than the inside, cracks are formed, which begin on the outside and gradually extend inward. These cracks are largest and most numerous at the ends of the log, where the drying is most rapid, and they are the cracks which we have already noticed in our boards.

Secondly, when timber has been cut up, if by any means one side of a piece is prevented from drying as fast as another, the side which dries most rapidly, and therefore shrinks most rapidly, becomes hollow, or the piece "warps." Or, if one side of a piece of wood which has been dried or "seasoned' is exposed to moisture, that side swells and becomes convex, and again the piece warps. Verify these statements by experiment, laying several pieces of board six or eight inches wide and of about the same length on the ground for some hours, or even on your bench if they have not been very well seasoned, setting up others on their edge so that both sides may be equally exposed to the air, and noting carefully the results after several hours. In the same manner, if wood has been already warped, it may be straightened by exposing it in the proper way.

(Samples of round timber stripped of the bark should be exhibited, showing the checks on the surface, and particularly at the ends, as well as one sample of a short log, cut up into boards, showing the cracks in the ends, and the edges of the boards, and in the faces of the outside boards or "slabs." The pupils should be made to observe for themselves the position and direction of these cracks in boards cut from different parts of the log. They should be made to observe how checking and warping continue after wood has been made up, if it is exposed, and how they are prevented by painting or varnishing).