When kiln-dried, the misleading moisture factor of weight is uniformly reduced, and a fair comparison possible. For the sake of convenience in comparison, the weight of wood is expressed either as the weight per cubic foot, or, what is still more convenient, as specific weight or density. If an old long-leaf pine is cut up (as shown in [Fig. 21]) the wood of disk No. 1 is heavier than that of disk No. 2, the latter heavier than that of disk No. 3, and the wood of the top disk is found to be only about three fourths as heavy as that of disk No. 1. Similiarly, if disk No. 2 is cut up, as in the figure, the specific weight of the different parts is:

showing that in this disk at least the wood formed during the many years' growth, represented in piece a, is much lighter than that of former years. It also shows that the best wood is the middle part, with its large proportion of dark summer bands.

Fig. 21. Orientation of Wood Samples.

Cutting up all disks in the same way, it will be found that the piece a of the first disk is heavier than the piece a of the fifth, and that piece c of the first disk excels the piece c of all the other disks. This shows that the wood grown during the same number of years is lighter in the upper parts of the stem; and if the disks are smoothed on the radial surfaces and set up one on top of the other in their regular order, for the sake of comparison, this decrease in weight will be seen to be accompanied by a decrease in the amount of summer-wood. The color effect of the upper disks is conspicuously lighter. If our old pine had been cut one hundred and fifty years ago, before the outer, lighter wood was laid on, it is evident that the weight of the wood of any one disk would have been found to increase from the center outward, and no subsequent decrease could have been observed.

In a thrifty young pine, then, the wood is heavier from the center outward, and lighter from below upward; only the wood laid on in old age falls in weight below the average. The number of brownish bands of summer-wood are a direct indication of these differences. If an old oak is cut up in the same manner, the butt cut is also found heaviest and the top lightest, but, unlike the disk of pine, the disk of oak has its firmest wood at the center, and each successive piece from the center outward is lighter than its neighbor.

Examining the pieces, this difference is not as readily explained by the appearance of each piece as in the case of pine wood. Nevertheless, one conspicuous point appears at once. The pores, so very distinct in oak, are very minute in the wood near the center, and thus the wood is far less porous.

Studying different trees, it is found that in the pines, wood with narrow rings is just as heavy as and often heavier than the wood with wider rings; but if the rings are unusually narrow in any part of the disk, the wood has a lighter color; that is, there is less summer-wood and therefore less weight.