TIMBER PHYSICS SOUTHERN PINE. 



351 



Logs from the top can usually be recognized by the larger percentage of sap wood and the smaller proportion 

 and moie regular outlines of the bands of summer wood, which are more or less wavy in the butt logs. 



The variation of weight is well illustrated in the foregoing table, in which the relative values are indicated in 

 italics. For comparison the figures for stiength of long-loaf 

 pine are added. 



Both weight and strength vary in the different parts of 

 the same cross section from center to periphery, and though 

 the variations appear frequently irregular in single individuals, 

 a definite law of relation is nevertheless discernible in large 

 averages, and onee determined is readily observable in every 

 tree. 



A separate inquiry, avoiding the many variables which 

 enter in the mechanical tests, permits the following deduc- 

 tions for the wood of these pines, and especially for long 

 leaf, the data referring to weight, but by inference also to 

 strength : 



1. The variation is greatest in the butt log (the heaviest 

 part) and least in the top logs. 



2. The variation in weight, hence also in strength, from 

 center to periphery depends on the rate of growth, the heavier, 

 stronger wood being formed during the period of most rapid 

 growth, lighter and weaker wood in old age. 



3. Aberrations from the normal growth, due to unusual 

 seasons and other disturbing causes, cloud the uniformity of the 

 law of variation, thus occasionally leading to the formation of 

 heavier, broad-ringed wood in old, and lighter, narrow-ringed 

 wood in young trees. 



4. Slow-growing trees (with narrow rings) do not make 

 less heavy, nor heavier, wood than thriftily grown trees (with 

 wide rings) of the same age. (See fig. 92.) 



EFFECT OF AGE. 



The interior of the butt log, representing the young sap- 

 ling of less than 15 or 20 years of ago, and the central portion 

 of all logs containing the pith and 2 to 5 lings adjoining is 

 always light and weak. 



The heaviest wood in long-leaf and Cuban pine is formed 

 between the ages of 15 and 120 years, with a specific weight of 

 over 0.60 and a maximum of 0.66 to 0.68 between the agea of 40 

 and 60 years. The wood foimed at the age of about 100 years 

 will have a specific weight of 0.62 to 0.63, which is also the 

 average weiglrt for the entire wood of old trees. The wood 

 formed after this age is lighter, but does not fall below 0.50 

 up to the two hundredth year ; the strength varies in the same 

 ratio. 



In the shorter-lived loblolly and short leaf the period for 

 the formation of the heaviest wood is between the ages of 

 15 and 80, the average weight being then over 0.50, with a 

 maximum of 0.57 at the age of 30 to 40. The average weight 

 for old trees (0.51 to 0.52) lies about the seventy-fifth year, 

 the weight then falling off to about 0.15 at the age of 140, 

 and continuing to decrease to below 0.38 as the trees grow 

 older. 



That these statements refer only to the clear portions of 

 each log, and are variably affected at each whorl of knots (every 

 10 to 30 inches) according to their size, and also by the variable 

 amounts of lesin (up to 20 per cent of the diy weight), must 

 be self-evident. 



Sapwood is not necessarily weaker than heartwood, only 

 usually the sapwood of the large-sized trees we are now using 

 is lep resented by the narrow-ringed outer part, which was 

 formed during the old-age period of growth, when naturally 



lighter and weaker wood is made; but the wood formed during the more thrifty diameter growth of the first 

 eighty or one hundred years— sapwood at the time, changed into heartwood later— was, even as sapwood, the 

 heaviest and strongest. 



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"Fm. 92.— Schematic section through stem of long leaf pine, 

 showing variation of specific weight, with height, diame- 

 ter, and age, at 20 (aba), 60 (ded), 120 (eece), 200 (ffff) 

 years. 



