24 



EFFECT OF MOISTURE. 



It will at once be evident from a casual inspection of tbese curves that all kinds of strength 

 (except tensile strength, which seems to be independent of moisture) increase greatly as the 

 moisture diminislies, and that this increase becomes very rapid after the timber has become, say, 

 half dry, or below 20 per cent moisture. Not only is the timber stronger, but it is more elastic. 

 Thus the gain in relative elastic resistance between 30 and 15 per cent of moisture is by a much 

 greater proportion than tlie corresponding gain in the elastic limit strength. The modnhis of 

 elasticity is also greater, so that we may say tiiat tlie drier the timber (within practical limits) 

 the stronger, the stififer, and the tougher it bec<mies. 



Since the si>eciflc gravity does not appreciably change as the timber dries ciut from 30 to 15 

 per cent of moisture (computed on the dry weight), it follows that the shrinkage in \dlume is 

 practically equal to the diminution of moisture, so that the weight of unit volume remains prac 

 tically constant in this species of timber. 



RELATION BETWEEN STRENGTH AND STIFFNESS. 



On Plate X are ])latted the average values of the moduli of elasticity for whole trees and 

 of the moduli of cross-beudiug strength at tlie elastic limit. The relation of these two curves 

 represents the relation \shi3h exists between the working strength and stiffness of a beam. It 

 appears that the stittiiess is a true index of the strenglli. and that in tiiis species the stift'er the 

 beam the stronger it will prove. The equation of tlie mean line as drawn on this jdate is — 



Elaxt if- limit nioihihts of sfrcnf/fh in ryonH-hreakiiKj = 5(10 + 0.0047 U (1) 



Where £' = niodulus of elasticity as found from a cross-bending test by making 



I]= J!^ . • . . . . (2) 



Where 17= concentrated load at center of beam. 

 I = length of beam. 

 /) = deflection of beam under load IT. 

 li = breadth of beam. 

 // = height of beam. 

 All dimensions being in inches and the load in jmunds. 



RELATION BETWEEN STRENGTH AND SPECIFIC GRAVITY. 



Plate XI exhibits the relation found to exist between crushing strength and specific gravity 

 or weiglit. Assuming this cuiv(^ to be a parabola, its ecpiation would be — 



Endwisrcnixhiiu/ st irii(/th =4:,0i)0 + 0,300 Vl^Hrav. — . 47 . . (3) 

 The diagram (Fig. 1-') shows the relation between the "average (jnality'' of the tind)er 

 and itssi)ecific gravity. In tiiis case each <piality was reiu'esented by a jjercentage of its own 

 average, and tlien the average of all those percentages was taken as -'average (piality" of eaeli 

 tree in eonii>arison with the "average quality" of all other trees. Here, also, a regular increas« 

 in average strength with an increase in spei-itic gravity is indicated. It was to be ])resunled lliat 

 this would l)e the case, wlien the results are all reduced to a standard dryness, since then tlie 

 weight ])er unit volume is a true measure of the amount of woody fiber and resinous matter in 

 the timber, and those in the same species determine to some extent the strength. 



RANGE OF INDIVIDIIAI, RESITLTS AS fJOMPARED WITH THE AVERA(iE OF ALL. 



I 



In Fig. i;i is shown a series of cuives which indicate tlie number of results ot each kind of test 

 falhng within given limits. Thus, tlie most accordant results were those of the endwise crushing 

 tests, over one third of which (150 in 430) fell between 05 jier cent and 105 ])cr cent of the mean 

 of all, while none of Ihem fell below !iO ])er cent or above 140 ]ier cent of the mean, wherea.'^iu 



