The fact that timber is not a manufactured material like iron or cement but is a natural product which has been formed by years of growth in the open where it has been all the while exposed to various adverse conditions of wind and weather, make it peculiarly liable to defects of different kinds, most of which can not be corrected and which render much of it unsuitable for use in construction. Moreover timber is not homogeneous like iron and steel products, in other words, it can not be safely assumed that several pieces of timber, even if they are cut from the same log, will have similar characteristics or will act in nearly the same way under the same conditions. Each piece of timber must be judged by itself and must be subjected to a very careful inspection if it is to be used in an important position with satisfactory results. Such inspection will often reveal some hidden weakness or blemish which sufficient to warrant the rejection of the piece as not good enough for the particular purpose for which it is intended,and such weaknesses or blemishes are known as defects.

Most of the defects which render timber unsuitable for building purposes arc due to irregularities in the growth of the tree from which the timber has been taken. These defects are known by various names as "heartshakes," "windshakes," "starshakes," and "knots." Other defects are due to deterioration of the timber after it has been in place for some time or even before the tree has been felled, among which are "dry rot" and "wet rot." The defects of the first class are defects of structure; those of the second class are defects of the material itself. It may also be said that the defects of the first class are permanent and are definitely defined, being caused by outside forces or conditions, thus the timber affected can be cut out and discarded leaving the rest of the piece perfectly sound and good, as the defect does not influence the timber near to it and does not spread. On the other hand the defects of the second class are in the nature of a disease which spreads from one part of a piece of timber to another and can even be carried from one piece of timber to another by contact.

Fie. 3. Blocks Showing Cross Grained, Partially Cross Grained, and Straight Grained Wood.

Fie. 3. Blocks Showing Cross Grained, Partially Cross Grained, and Straight Grained Wood.