HOW TO INCREASE DURABILITY. 57 



Perfect seasoning, preferably kiln-drying, before using, and protec- 

 tion against the entrance of moisture by tar, paints, and other covers, 

 when put in place, prolong the life of wooden structures. Where such 

 a covering is too expensive, good ventilation at least is necessary. 

 Contact surfaces, where timber rests on timber or brick, should in all 

 cases be especially protected. 



Different species differ in their resistance to decay. Cedar is more 

 durable than pine and oak better than beech, but in most cases the con- 

 ditions of warmth and moisture in particular locations have so much 

 to do with durability that often an oak post outlasts one of cedar, even 

 in the same line of fence, and predictions of durability become mere 

 guesswork. 



Containing more ready-made food, and in forms acceptable to a great 

 number of different kinds of fungi, the sapwood is more subject to 

 decay than the heartwood, doubly so where the latter is protected by 

 resinous substances, as in pine and cedar. Several months of immer- 

 sion improves the durability of sapwood, but only impregnation with 

 preservative salts seems to render it perfectly secure. Once attacked 

 by fungi, wood becomes predisposed to further decay. 



Wood cut in the fall is more durable than that cut in summer, only 

 because the low temperature of the winter season prevents the attack 

 of the fungi, and the wood is thus given a fair chance to dry. Usually 

 summer-felled wood, on account of prevalent high temperature and 

 exposure to sun, checks more than winter-felled wood, and since all 

 season checks favor the entrance of both moisture and fungus, they 

 facilitate destruction. Where summer felled wood is worked up at 

 once and protected by kiln-drying no difference exists. The phases of 

 the moon have no influence whatever on durability. 



In sawing timber much of the wood is bastard cut; at these places 

 water enters much more readily, and for this reason split and 

 hewn timber and ties generally resist decay perhaps better than if 

 sawed. 



The attacks of beetles, as well as those of the shipworm, can not here 

 be considered; like chisel or saw they are mechanical injuries against 

 which none of our woods are proof. 



Range of durability in railroad ties. 



Tears. Years. 



White oak and chestnut oak 8 



Chestnut 8 



Black locust 10 



Cherry, black walnut, locust 7 



Elm 6 to 7 



Red and black oaks 4 to 5 



Ash, beech, maple 4 



Red wood 12 



Cypress and red cedar 10 



Tamarack 7 to 8 



Lou {f leaf pine 6 



Hemlock 4 to 6 



Spruce 5 



The durability of wood, exposed to the changes of the weather, and 

 where painting, after thorough seasoning, is impracticable, is increased 



