Chap. XXII.

TIMBER AND ITS PROPERTIES.

1289

and before wheels were rung with iron, this sort of willow was employed for the fellies,enduring under friction and exposure a longer time than most other woods.

Felling Timber. — To the timber-grower the object aimed at should be that of obtainingthe largest quantity of hard and durable wood, as free from sap as possible : no tree shouldbe severed from its stool until it has arrived at a state of maturity, and previous to its beingthrown down, the natural juices which pervade it should be discharged and allowed to runout, that the wood may be freed from what would bring about premature decay, andprevent its becoming dry and hard.

Barking the tree whilst in a growing state, and cutting through the sop or sap-wood, soas to allow the juices to be discharged, are practices of high antiquity. Vitruvius observesthat both the density and the strength of timber are much improved by causing the tree to diestanding : and most persons have agreed that barking the timber, previous to felling, greatlyimproved its quality. “ When a tree has an incision made through the sap-wood atits trunk it soon dies, and no further change takes place ; and the barking of trees, whenin full growth and vigour, and letting them stand a twelvemonth after the operation, isadmitted not only to improve the quality, but to increase the quantity, at the same timethat it seasons the wood : time, however, is the best seasoner, and no artificial method canequal the natural process, which is that of getting rid of all juices in so regular a manner,that dissipating them does not too rapidly shrink and crack the timber. Trees may besuffered to stand too long before they are cut down ; but this is not a usual fault, for oftenwhen they can measure to a load, which in half a century is sometimes the case, they aremarked for felling, the heartwood not having either acquired its hardness or its fullquantity.”

Seasoning Timber. —Nothing more effectually contributes to this purpose than sufferingit to lie for a time in fresh water ; and our fir timber on its arrival is usually formed intofloats on the Thames, where it remains for some time; thus the natural juices of the treeare removed by the water penetrating throughout its pores; this not containing anyquality to produce fermentation, and being afterwards easily evaporated, tends to improveit for building purposes. Salt water, on the contrary, would require a considerable timeto evaporate from the wood, in consequence of its deliquescence, and would render it unfitfor use : when taken out of the water, it should be suffered to become thoroughly dry beforeit is carried to the pit to be sawn ; and the usual method adopted is to block it up from theground, and suffer a free circulation of air around it. When the tree is cut into scantlings,a further seasoning should be permitted by a free exposure to air, guarding against tooviolent an effect of wind or the sun's rays, and if it be cut into boards, they should be piledone on the other, with fillets or small pieces of wood between them, or laid in a triangularform, with their ends alternating, so as to permit the air to pass freely about them.

Gradual drying should always be resorted to, as it invariably produces the most durabletimber, the object of seasoning being chiefly to drive off the water, and not to disturb anyportion of the carbon, which would certainly be the consequence of elevating the tempera-ture of the wood, or exposing it to great heat. Timber dried too quickly loses its tough-ness as well as its pliability; the outer pores become rapidly contracted, and do not permitthe moisture from within to pass off* freely.

Experiment has proved that oak, when seasoned properly, has lost nearly two-fifths of itsweight, showing the necessity of allowing a sufficient time for the juices to be thoroughlydriven off, which time must vary in proportion to the scantling of the timber ; if cut intothin planks the operation of seasoning would be perfected in quicker time than thatwhich would be necessary for the entire tree. All timber should be permitted to arrive atthis dry state before it is cut into smaller scantlings or planks, great shrinking being the con-sequence of their suddenly drying by exposure to the air, which, by disturbing the fibres ofthe wood, affect its durability : to avoid this it was formerly the practice, after the plankswere cut out by the sawyer, to prevent their exposure to the air, by laying them imme-diately in a running stream, and after some time to dry them very gradually, care beingtaken that the planks, when immersed, were removed from atmospheric influence; but,like steaming or boiling, this injured the strength of the wood, though it prevented itswarping, and rendered it easier to work. The changes produced by these artificial processesundoubtedly injure the fabric of the wood ; when a piece of thoroughly seasoned timber isimmersed in water, and afterwards subjected to the heat of a stove, it will weigh less thanpreviously ; consequently a portion of its carbon must have passed off*.

here posts or piles are to be driven or plunged into the earth, it has been from theearliest time customary to char the surface to be placed underground: but to apply thispractice to green timber is injurious, as it confines within, rather than expels thesejuices, which by fermenting cause its decay; it may be serviceable in destroying anyfungi or worms that may attach themselves, or prevent their preying upon the fibres of thewood.

Decay of Timber, its Causes. — As woods were supposed to differ in the proportions oftheir ultimate elements, various experiments have been made upon them. Dr. Prout hasshown satisfactorily that their composition is similar, and that they consist of equal weights