I-] 



ANNUAL RINGS. 



il 



engineering purposes^ depends, to a great extent^ upon 

 the degree of firmness and solidity of the annual layers. 

 These layers are all very plainly marked in the Oak and 

 Fir, and in most European woods ; but in the Maple 

 and Lime, and in some others, they are less obvious, 

 while in many trees of tropical growth they are so in- 

 distinct that it is impossible to trace them. 



The woody layers, when first formed, are full of sap, 

 but they change and gradually become solidified by the 

 thickening and drying of the wood-cell walls of each 



FIG. 4. 



FIG. S. 



subsequent layer, and their infiltration with various 

 preservative and other materials; and as each zone is 

 moulded upon ;one of the previous year's growth by 

 the action of the continuous cambium, it must, by 

 cohesion, be amalgamated with it. The perfecting of 

 the concentric layers is, however, a very gradual process 

 and the time necessary to convert a new layer of sap- 

 wood into heart-wood (which alone represents the 

 serviceable timber in most trees) varies from about one 

 year to thirty years or even more. It seems, as a rule, 

 from evidence to be shown later on in Table I., that 



