Rothrock.] 600 [Feb. 2. 



So for as the ducts are concerned, Avhile the material of -which they are 

 composed may be quite as strong as that of the fibres, yet owing to the 

 enormous cavity they contain, it is apparent that as compared Avitli fibres, 

 tliey must be much wealier ; that in fact every duct is to be regarded as an 

 element of weakness to the stick. Hence then, other things being equal, 

 the more fibres and the fewer ducts, the stronger is any given stick of tim- 

 ber as compared with another of the same species. 



The question of durability in exposed positions is quite another thing, and 

 has no close relation to strength. 



Accepting the above facts as proven, mere examination of a cross section 

 of timber with the naked eye, or at most with an ordinary hand lens, may 

 afford a reasonably safe way of estimating the quality of a given specimen 

 of wood. 



Associated with the appearance presented by the ducts, and the mass of 

 fibres, is another element of structure, i. e., that of the annual rings. These 

 are usually caused as may be seen (A and B G) by the thick, flat cells which 

 are formed in autumn as contrasted (A and B 7) with the larger ones which 

 mark the first growth of the ensuing spring. The number of rows which 

 are thus flattened in the autumn wood is by no means constant. Sometimes, 

 as in the case of the White Oak, there being but two, three or four ; or as 

 in the case of the Chestnut being often about eight, or more ; or as in the 

 Redwood of California (Sequoia sempervirens) as high as fifteen. As a rule 

 the color in all these autumn fibres is deeper than in those made earlier. 

 Hence both shape and color combine to mark the "year's growth." 



The term "year's growth" is one which should not be depended upon 

 too absolutely, inasmuch as it is well known to be misleading at times. 

 Thus, in the American Linden, one frequently sees a ring more on one side 

 than on the other ; and indications are not wanting, which would prove 

 that very frequently several such rings may form in our latitude in a single 

 season. 



There are some focts of practical importance connected with the wood 

 formed during the season, or to speak more accurately, with all the tissue 

 lying between the denser, flatter fibres which are assumed to be formed in 

 the autumns of two successive years. In White Oak, as shown by figures 

 A and B, there may be a great range in the distance between these zones 

 of flat fibres. Thus fig. B shows that the growth for the year was about 

 twice that shown by fig. A. The former of these figures represents a 

 good specimen of White Oak, and the latter a bad one, each having been 

 carefully tested for strength by competent mechanical experimenters. In 

 these instances the reason for the difference in the quality of the wood is 

 obviously in the relative predominance of solid woody fibre over open ducts 

 in the good specimen (B), and the lesser quantity of wood as compared 

 with ducts in (A), the bad. It so happens that in A the diameter of the 

 duct (.01430 of an inch) is greater by far than in the better wood. This can, 

 however, hardly be regarded as constant. What does appear to prevail in 

 White Oak is, the fact that most of these large ducts are made early in the 



