26 



TIMBER. 



Separating a single cell, as shown in fig. 1G, a, drying and then drop- 

 ping it into water, it floats. The air-filled cell cavity or interior reduces 

 its weight, and, like a corked empty bottle, it weighs less than the water- 

 Soon, however, water soaks into the cell, when it fills up and sinks. 



Many such cells grown together, as in a block of wood, sink when 

 all or most of them are filled with water, but will float as long as the 

 majority are empty or only partly filled. This is why a green, sappy 

 pine pole soon sinks in "driving" (floating). Its cells are largely filled 

 before it is thrown in, and but little additional water suffices to make 

 its weight greater than that of the water. 



In a good-sized white pine log, composed chiefly of empty cells (heart- 

 wood), the water requires a very long time to fill up the cells (five years 

 would not suffice to fill them all), and therefore the log may float for 

 many months. When the Avail of the wood fiber is very thick (five- 

 eighths or more of the volume), as in fig. 16, b, the fiber sinks whether 

 empty or filled. This applies to most of the fibers of the dark summer- 

 wood bands in pines, and to the compact fibers of oak or 

 hickory, and many, especially tropical woods, have such 

 thick walled cells and so little empty or air space that they 

 never float. 



Here, then, are the two main factors of weight in wood : 

 The amount of cell wall, or wood substance, constant for any 

 given piece, and the amount of water contained in the 

 wood, variable even in the standing tree, and only in part 

 eliminated in drying. 



The weight of the green wood of any species varies chiefly 

 as the second factor, and is entirely misleading if the rela- 

 tive weight of different kinds is sought. Thus some green 

 sticks of the otherwise lighter cypress and gum sink more 

 readily than fresh oak. 



The weight of sapwood, or the sappy peripheral part of 

 our common lumber woods, is always great, whether cut 

 in winter or summer. It rarely falls much below 45 pounds 

 and commonly exceeds 55 pounds to the cubic foot, even in our lighter 

 wooded species. 



It follows that the green wood of a sapling is heavier than that of an 

 old tree, the fresh wood from a disk of the upper part of a tree often 

 heavier than that of the lower part, and the wood near the bark heavier 

 than that nearer the pith, and also that the advantage of drying the 

 wood before shipping is most important in sappy and light kinds. 



When kiln dried, the misleading moisture factor of weight is uni- 

 formly reduced and a fair comparison possible. For the sake of con- 

 venience 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 longleaf pine is cut up as shown in fig. 17, the 

 wood of disk No. 1 is heavier than that of disk No. 2, the latter heavier 



a, 3 



Fig. 16.— Iso- 

 lated fibers. 



