The force which causes water to rise through the trunk of a tree is not fully understood. It is one of nature’s mysteries which is yet to be solved. Forces known as root pressure, capillary attraction, and osmosis, are believed to be active in the process, but there seems to be something additional, and no man has yet been able to explain what it is. The seasoning of wood is the process of getting rid of some of the water. As soon as lumber is exposed to air, the water begins to escape. Long exposure to dry air takes out a large percentage of the moisture which green wood holds, and the lumber is known as air-dry. But some of the original moisture remains, and air at climatic temperature is unable to expel it. The greater heat of a drykiln drives away some more of it, but a quantity yet remains. The lumber is then kiln-dry. Greater heat than the drykiln’s is secured in an oven, and a little more of the wood’s moisture is expelled; but the only method of driving all the moisture out is to heat the wood sufficiently to break down its structure, and reduce it to charcoal. Wood warps in the process of drying unless it seasons equally on all sides. It curls or bends toward the side which dries most rapidly. Dry wood may warp if exposed to dampness, if one side is more exposed and receives more moisture than another. It curls or bends toward the dryer side. Warping is primarily due to the more rapid contraction or expansion of wood cells on one side of the piece than on the other. Saturated cells are larger than dry ones. Moisture in wood affects its strength, the dryer the stronger, at least within certain limits. Architects and builders carefully study the seasoning of timber, because it is a most important factor in their business. The moisture which most affects a wood’s strength is that absorbed in the cell walls, rather than that contained in the cell cavities themselves. Some woods check or split badly in seasoning unless attended with constant care. Checking is due chiefly to lack of uniformity in seasoning. One part of the stick dries faster than another, the dryer fibers contract, and the pull splits the wood. The checks may be small, even microscopic, or they may develop yawning cracks such as sometimes appear in the ends of hickory and black walnut logs. Greenwood checks worse in summer than in winter, because the weather is warmer, the wood’s surface dries faster, and the strain on the fibers is greater. Phases of the moon have no influence on the seasoning, checking, warping, or lasting properties of timber.