Page:Popular Science Monthly Volume 2.djvu/606

586 will be pulled round or bent convex, while the outside will be the reverse, or hollow, and the plank will be considerably narrower throughout its entire length, more especially on the surface of the hollow side. Selecting the next two planks, they will be found to have lost none of their thickness at the centre, and very little of their thickness at the edges, but very much of their breadth as planks, and will be curved round on the heart-side and made hollow on the outside. Supposing some of these planks to be cut up into square prisms when in the green state, the shape that these prisms will assume after a period of seasoning will entirely depend on the part of the tree to which they belong, the greatest alteration would be perpendicular to the medullary

rays. Thus, if the square was originally near the outside, as seen in Fig. 3, then the effect will be as shown in Fig. 4, namely, contraction in the direction from a to b. After a year or two the square end of the prism will become rhomboidal, the distance between c and d being nearly the same as at first, but the other two edges brought closer together by the amount of their contraction. By understanding this natural law, it is comparatively easy to predict the future behavior



of a board or plank by carefully examining the end-wood, in order to ascertain the part of the log from which it has been cut, as the angle of the ring-growths and the medullary rays will show this, as in Figs. 5 and 6. If a plank has the appearance of the former, it must have been cut from the outside, and for many years it will