BUILDING MATERIALS. 255 



10. Timber. Timber holds an important position in the construction 

 of buildings, and it is of great consequence to choose timber suitable for 

 the different purposes required. The timber of a tree may be put down 

 as consisting of two parts namely, the sap-wood and the heart the 

 sap-wood being that next the bark, and the heart-wood the inner portion ; 

 the latter is much more durable, firm, and compact than the former, 

 and less liable to decay. 



Generally speaking, timber should never be used with the bark on, as 

 it tends to assist in the decay of the timber enclosed within it. 



The seasoning of timber should be attended to before it is used for 

 building. Trees cut for this purpose should not be felled while the sap 

 is in full flow. The best months for this work are November, De- 

 cember, January, and February. If timber is used in a green state, it 

 not only is very liable to decay, but it is apt to shrink and split up very 

 much ; and when this takes place in a building, the consequences are 

 often very serious indeed. Trees are either seasoned naturally or arti- 

 ficially. The natural system is to expose them to drying winds in 

 some place where they are kept dry and away from the sun's rays. This 

 system of seasoning is by far the best which can be adopted ; but it 

 takes a long time to do this, as most trees take nearly two years to 

 dry properly. In shipbuilding -yards the timber is often artificially 

 dried by steam. This makes the green timber less liable to decay, but 

 the strength of the timber is deteriorated. It is a very quick process, 

 and is useful in causing the timber to bend for shipbuilding and boat 

 purposes. 



The best timber is got from trees which grow on exposed and dry 

 situations, those being more inferior in quality which grow on sheltered 

 and moist situations. This is fully exemplified in the natural forests of 

 Scots pine in Scotland, such as exist in Strathspey, and at Invercauld in 

 Aberdeenshire. In these cases, the trees growing on the highest parts 

 of the forests and the most inferior soils are found to produce the best 

 timber. The growth of those trees has, of course, been very slow. I 

 have numbered as many as sixty annual rings or growths to an inch 

 in the diameter of a tree. Such trees are full of pitch, and are very 

 compact and durable. 



In many cases the quality of the timber alters very much with the 

 character of the tree itself. Thus, timber of a straight grain and free 

 from knots is, as a building material, much superior in strength to that 

 which is twisted in the grain and full of knots. 



The two great evils the builder has to contend with in timber are its 

 liability to dry and wet rot. The cause of these rots is fermentation, 

 which is followed by putrefaction of the tnnber. The dry-rot gener- 



