-A NOTES OX THE PROPERTIES OF WOOD. 



outer side ; this causes the splits on the exterior, after the wood is dry, 

 and it also accounts for the impossibility of forming out of green wood a re- 

 gular body which shall not lose its weight or shape. The time during 

 which the evaporation of the moisture takes place depends upon the 

 state of the atmosphere. Several authors are of opinion that the con- 

 traction is regulated by the specific weight. This, as a rule, cannot be 

 adopted ; for instance, lilac and oak, being both heavy and hard, con- 

 tract quickly, whilst the double American maple, which is equally hard, 

 contracts slowly. It is evident, however, that the fuller of sap a tree is, 

 the greater will be the contraction. This will account for the fact, that 

 we find on cut timber the cracks extend from the exterior towards the 

 centre, because the sap-wood will contract more than the heart-wood. 



Planks turn with their sides upwards ; that is, the edges rise from 

 the level of the centre line. This explains why we turn the inside of a 

 plank towards the joists whilst we lay a floor, which prevents the twist- 

 ing. The contraction and action of the wood cannot be checked 

 altogether. Among the means to prevent it stands foremost the one of 

 putting the tree into water ; but wdien taken out it must not be stored 

 in a place where it dries too soon, as it would burst if done ; nor must 

 it be left too long in the water, as this will injure the quality of the 

 wood. 



All the wood which is used is never quite dry ; besides this, it 

 works a little in itself under the changes of the atmosphere ; and there- 

 fore attention must be paid to the selection of timber for the same 

 purpose, for the heart-wood is less subject to such action than sap or 

 splint-wood. This is an important point with the furniture makers. 

 The expansion or contraction is less apparent in the length of the fibre 

 than in the cross section of the wood. 



The elasticity of the wood is called its property of return to its 

 original form and shape after these have been altered by another cause. 

 "With this quality is connected the valuable fitness of some woods to 

 transmit sounds, for which reason they are much used hj musical in- 

 strument makers. The elasticity has its limit ; and the more a piece of 

 wood can be bent without breaking, the greater its elasticity. 



The resistance of wood depends upon the three following conditions ; 

 — 1st. Two pieces of equal length and height offer different resistance 

 with reference to their breadth. 2nd. Two pieces of equal length and 

 breadth offer different resistance with regard to their height. 3rd. Two t 

 pieces of wood of equal height and breadth offer different resistance with 

 regard to their length. 



Again, experience has proved that a piece of wood resting freely with 

 its two ends will carry from one-third to one-half less weight than if the 

 ends are fixed or built in a wall. Old wood has less elasticity than 

 young, and heart-wood is less elastic than sap-wood. The same differ- 

 ence is found with wood from the lower stem compared with that from 

 the upper crown. Th<r elasticity diminishes with the progress of the 



