Fig. 9.
You have, of course, seen the rings, or circular lines, on the ends of pieces of wood (Fig. 9). These are called the annual rings,[8] and each ring marks a new layer of wood added to the tree, for, as perhaps you may have learned, the trees we use for wood-working grow by adding new layers of wood on the outside. Examine the ends of pieces of wood of various kinds. In some pieces these rings will be very plain. In others they will be quite indistinct.
Notice that the wood nearest the bark, known as the sapwood, usually looks different from the inner wood, which is called the heart (Fig. 9).
In some trees you will see rays, or lines, radiating from the centre, and known as the medullary rays (Figs. 9 and 10), because they spring from the pith (Latin medulla). Sometimes these lines are too fine to be noticed.
You will see from Fig. 10 that the layers of wood are also shown in the lines of what we call the "grain" on the surface of a piece of wood cut lengthways, and that the lines of the grain are continuations of the annual rings. You will also notice at the ends of timber, after the seasoning has begun, cracks radiating from the centre, showing the natural lines of cleavage or separation.
Fig. 10.
Fig. 11.
The way the log is sawed is important, though you might naturally think that the only thing is to saw it any way that will give pieces of the required size and shape.