In addition, the knots caused by branches, the twisting of the stems screw-fashion (as is seen in cedar), wounds, and other causes, often produce very crooked and tangled grain, and the wood of many broad-leaved trees is sometimes extremely complicated in texture, especially when all these irregularities occur in the same piece. It is the nature of some kinds of mahogany, from whatever cause, to have the fibres strangely interlaced or running in very different directions in layers which are quite near each other.

The warping, twisting, and cracking is obviated in many cases where it is objectionable (as in the wooden frames of machines, the tops of benches) by building up with a number of smaller pieces, of which you will often see illustrations. To do this to the best advantage, the pieces should be selected and put together so that, though the grain will run in the same direction lengthways, the annual rings at the ends will not run together as in a whole beam, but will be reversed or arranged in various combinations, so that the tendencies of the different parts to warp or twist will counteract each other. Instead of a single board, which would naturally become warped in one large curve, a number of strips can be glued up with the grain of the strips arranged in alternate

Appendix fashion (Fig. 559), so that in place of one large curve the warping will merely result in a slightly wavy line.

Where but one side of a board is seen or used and where the full strength is not needed, warping and twisting can be largely prevented by lengthways saw-cuts on the back or under surface, as in a drawing-board, the crossways strength required being secured by the cleats. Doors and most forms of panelled work also illustrate these matters of swelling and shrinking (see Doors and Panels). Shakes. - Heart-shakes are cracks radiating from the centre in the line of the medullary rays, widest at the pith and narrowing toward the outside, and supposed to be chiefly caused by the shrinkage of the older wood due to the beginning of decay while the tree is standing (Fig. 702). Slight heart-shakes are common, but if large and numerous or twisting in the length of the log, they injure the timber seriously for cutting up.

Star-shakes are also radiating cracks, but, unlike the heartshakes, the cracks are widest at the outside, narrowing toward the centre (Fig. 703), and are often caused by the shrinkage of the outer part due to the outside of the tree drying faster than the inside, as it naturally does from being more exposed after being felled; but they are sometimes owing to the beginning of decay and other causes.

Cup-shakes are cracks between some of the annual rings, separating the layers more or less (Fig. 704), sometimes reaching entirely around, separating the centre from the outer portion, and are supposed to be caused by the swaying of the tree in the wind (hence sometimes known as wind-shakes), or to some shock or extreme changes of temperature, or other causes.

Combinations of the various shakes may be found in the same log.

Fig. 702.

Fig. 703.

Fig. 704.

A Few Suggestions about Working - Drawings. Drawing is far too extensive a subject to be even briefly treated in a manual on wood-working, but a few general remarks on matters connected with working-drawings may be of help to some.

While an ordinary picture gives a correct idea of how an object looks, we cannot take accurate measurements from it. When we need dimensions, as in practical work, we must have some drawings which will show us at once the exact shapes, sizes, and positions of the various parts. In addition to the picture to give us the general idea, we have for working purposes what are called elevations, plans, sections, etc.

In such a case as that of the little house shown on page 242, the picture (Fig. 363) shows us the appearance of the building, but for purposes of construction, working-drawings should also be made. The view of what you would see if you stood directly in front of this house, with only the front visible, is shown in Fig. 364, and is called the front elevation. Stand opposite either side or end, and the view seen is represented in Fig. 364 as the side elevation. In the same manner the rear elevation is given. Next imagine yourself in the air directly above the house. This view is called the plan.x In this case, as the view of the interior is desired, the view is shown as if the roof were removed. If the sides or ends are not alike, as is sometimes the case, two side or end views may be needed. In the case illustrated, inside elevations are also given, to show the construction.

1 This definition of elevations and plan as being representations of what you would see if you stood opposite the sides or above the top of the object, is merely a rough explanation of the general meaning of the terms. As a matter of scientific accuracy the elevation is, strictly speaking, not the way the side would appear if you looked at it from one position, but the way it would appear if you could look at it from directly opposite every point of it - as if you could have an infinite number of eyes, one being opposite every point of the object. The elevation shows the front or side or end as it really is, not as it looks, either in the form of an exact copy if the object is small, or of a small copy made in the same proportion if the object is too large to be represented full size.