TIMBER: FORESTRY 393 



most typical of the ring-porous woods are ash and oak, and of 

 the diffuse-porous ones, birch and maple. 



In Figs. 316 and 317 the lower part of each section as it 

 stands on the page is the portion nearest the pith of the trunk 

 from which it came. How could this be known in the case of 

 Fig. 317? Assuming that close texture (shown by the darker 

 shade of the sections) accompanies slow growth, is the rate of 

 growth the same throughout the season ? If not, in which 

 wood is it most nearly the same, and in which is it most un- 

 equal? If the pine wood shown in Fig. 316 is (except the 

 resin passages) mostly made up of tracheids, when are the 

 largest tracheids formed ? 



363* Some physical properties of wood. All wood contains 

 moisture, the sapwood or outer portion more than the heart- 

 wood or inner portion. Not all of this water can be driven 

 off, but much of it is removed by seasoning in the open air, 

 and still more is expelled by drying lumber in kilns at a tem- 

 perature of from 158 to 180 F. Coniferous woods may be 

 placed in the kiln as soon as sawed, but the hard woods are 

 usually first seasoned in the air for from three to six months. 

 The loss of water from heartwood during kiln-drying amounts 

 to from 16 to 60 per cent of the weight of the green lumber, 

 the amount depending on the kind of wood treated. 



When kiln-dried the heaviest woods, such as hickory, oak, 

 and the closest-grained ash, weigh from 42 to 48 pounds per 

 cubic foot. Those of medium weight, such as southern pine, 

 sycamore, and soft maple, weigh from 30 to 36 pounds per 

 cubic foot. The very lightest, such as white pine, spruce, and 

 poplar, weigh from 18 to 24 pounds per cubic foot. Since 

 water weighs about 62 J pounds per cubic foot, all of our 

 native woods float when dry, though some of them sink 

 when green. . 



Wood is a very strong material in proportion to its weight. 

 A piece of hickory, good oak, or long-leaf pine, 1x1 inch and 

 12 inches long (loaded in the middle and supported at both 

 ends) requires a weight of about 720 pounds to break it. 



