FOREST UTILIZATION 61 



loaded to over one-fourth of its indicated 

 strength. 



Transverse strength is always proportioned to length 

 of girder; to width of girder; and to the square 

 of the depth of girder. It is the quality of tim- 

 ber which ' is most required in timber used for 

 building purposes. 

 VI. Hygroscopical qualities. 



(a) Timber changes form, coherence and volume with 



greater or lesser ease under the influence of moist- 

 ure, applied in gaseous or liquid form. -Hence 

 shrinking, swelling, warping, checking, cracking, 

 casehardening and working. 



(b) Water invariably saturates the cell walls; in addi- 



tion, it may or may only partially fill the lumina. 



(c) Sapwood invariably contains more water than heart- 



wood. 



(d) Rate of dryness depends on the species, looseness of 



tissue, dimensions of object to be dried, presence 

 or absence of bark cover in logs, preceding treat- 

 ment by floating, deadening, steaming, prevalence 

 of sapwood or heartwood, season of year, ex- 

 posure to wind, climate etc. 



(e) Boiling and steaming reduce the hygroscopicity and 



produce, consequently, a more even shrinkage. 



(f) The evaporation from the cross section bears to that 



of the tangential and to that of the radial section 

 the ratio of 8 to I to 2. 



(g) In the dry kiln, temperatures of 160 degrees to 180 



degrees Fahrenheit are gradually produced. Dry- 

 ing is accomplished by hot air, steam and moving 

 air. 



Conifers stand the dry kiln process much better 

 than hardwoods. The better qualities of hard- 

 woods undergo air drying before being kiln dried, 

 especially so in wagon, furniture and barrel fac- 

 tories. 



The dry kiln saves insurance and interest on large 

 stocks of lumber and allows the lumberman to 

 rapidly fill pressing orders for lumber, 

 (h) Wood is least permeable for water in the direction 

 of the tangent or vertically to the medullary rays 

 a fact important for tight cooperage. 

 I. Shrinkage. 



It is least along the fibre ; it is up to 5 % 

 along the radius^ and is up to 10 % along 

 the tangent. 

 Shrinkage of over 5 % of green volume 



