298 TIMBEES AND THEIR USES 



moisture percentage and the sap is of a 

 thick, viscous nature. When cut they 

 season very slowly and shrink to a com- 

 paratively small extent in the process. 



Timbers growing in loose " porous " districts 

 have a higher moisture percentage and the sap 

 is of a more fluid nature. They season more 

 rapidly, but shrink very little in the process. 



The straight-grained timbers season more 

 rapidly and shrink to a greater extent than 

 those with lower initial nioisture. 



Tests designed to show the ultimate strength 

 of wood may take several forms, and shordd be 

 so modified as to reveal the stress required to 

 produce rupture, under conditions approxi- 

 mating to those to which the wood will be sub- 

 jected during use. For example, in testing 

 Australian woods for use as sleepers, three items 

 are always considered. 



1. The durabiUty of the timber, and its 



capacity for resisting dry rot, etc. 



2. The hardness and toughness of the material 



to enable it to resist indentation by the 

 rail without tendency to " split " or 

 break up in service. 



3. The requisite capacity for retaining the 



" dog spikes " in place. 



