30 



explained by the fact tliat here the eell wmH.s are tliiuuer. Passing towards the heart, the strength 

 increases and reaches a niaxinium in llie butt of a mature tree at a distance of abont one-third of 

 the radius from the heart. In the immediate vicinity of tlie heart tlie strengtli decreases again. 

 In very old trees this strength may diminish to zero, the heart portion det-aying and wasting away 

 entirely. For the upper parts of a tree, the central heart portion is doubtless the strongest part 

 of the log. 



Tlie variation of strength is not great, however, as shown in Fig. 15, the range being only abont 

 12 per cent of the average strength, namely from 4 per cent below to 8 per cent above the mean 

 strength of all the sticks in this log. 



In crushing across the grain and in shearing there seemed to be no relation of variation of 

 strength and radial situation within the log exhibited. 



VARIATION OF STRENGTH AT DIFFERENT HEIGHTS FROM THE GROUND. 



In Fig. 1("> is shown the diminution in strength at increased distances from the base of the tree 

 under the ordinary kinds of stress. The modulus of elasticity (.stiffness) and the tensile strength 

 show greatest loss, each losing abont 40 per cent of their strength at a height of 70 feet. The 

 cross-breaking strength is reduced by one-third and the crushing endwise strength by only about 

 one- fifth in this distance. The first 20 or 30 feet of the tree seems to be of about a c(>Hstant strength. 



SHEARING AND CRUSHING STRENGTH PARALLEL AND TRANSVERSE TO THE ANNUAL RINGS. 



The shearing tests were made in two ]ilanes at right angles to each other on each stick. 

 Whenever these shearing planes came parallel and perpendicular to the annual rings, an opportu- 

 nity was offered for determining the difference in strength in these two ways. The mean results 

 from seventy-live such cases showed a shearing strength on planes parallel to the annual rings of 

 579 pounds per square iiu'h, and perpendicular to or across the rings of CKi pounds per square 

 inch, a difference of (1.2 per cent. In a similar way the crushing strength across the grain was 

 found to be (mean of 70 tests) 1,106 pounds per square inch, while parallel to the grain it was 1,257 

 ]>oiinds per square inch at theS per centdistortiou limit,or a slrength 12.8 percent greater parallel 

 to the annual rings. Tiicse percentages are both too small to take any account of in actual prac, 

 tice, and can therefore be ignored in this sjiecies of timber. 



* 



SHRINKAGE IN SEASONING.* 



Plate XI seems to indicate that the specific gravity remains practically constant as the per 

 centage of moisture diminishes from 30 to 15 per cent of the dry weight. This implies that the 

 diminution of volume is practically equal to the diminution of weight, so that the weight of unit 

 volume is about constant. This proba1.)ly holds true in all timbers for slight variations in the 

 percentage of moisture of seasoned lumber or for changes in moisture between 10 and 20 ])er cent. 

 Ordinary seasoned lumber, left out of doors, will retain about 15 per cent moisture, comjjuted on 

 the dry weight, while wood used indoors, as in house-finishing, furniture, etc., will retain alxiut 

 10 per cent. 



EFFECT OF BOXING OR BLEEDIN(J FOR TURPENTINE ON STRENGTH OF TIMBER. 



In a circular by the Forestry Division (No. 8) a |ircliminary statenu'nt oC results of tests on 

 timber bled for turpentine was made, which contained the following language: 



One seiifs of tests Wiis instituted t(i determine tlH-ett'ect wliicli the praetiir (ifgatherinf; rcsinons matter ('(n the 

 inannfaeture of turpentine and naval stores from the h>ng leaf pine of tlie South may liave upon the strength oltlic 

 timher of trees subjected to this practice. 



The gathering of resin is done by cutting a recess (box) into the foot of the tree, whieli is called "boxing" 

 the tree, and tlien scarring (chipping) the trunk above the box, increasing tlie size of tlie scar from year to year. 

 From this scar the semi-liquid resin exudates and drains into the box; this process is continued for four years nn<l 

 then the trees, lessening in yielil, are abandoned. 



- Special studies on tliis nuestiou are carried on in the physical laboratory by Mr. F. Roth, aud -veill be pub 

 lished as soon as completed. 



