11 



qnautities, ;ts iuclicated by the straight line drawn through the plotted points. The algebraic 

 expression ot the hiw shown by tliis line, rendered into pounds per square ineh, is, in round 

 numbers — 



Cross-breaking strengtli = 0.0045 Modulus of Elastieity + 450. (3) 



If it should be found that there is such a law for all kinds of tindier, then there nuiy lie derived 

 !in equation of this form, but with different constants, for each species. 



Relation heticeen strength and ireiyht. 



In Fig. 8 is shown the relation between the crushing strength and the specific gravity, when 

 both are reduced to the standard percentage of moisture, which was taken at 15 pev cent. 



These results are also taken from Professor Bauschinger's published records of tests on i)ine, 

 larch, and lir tindjers, and they conchn-ively show that the greater the weight the greater the 

 strength of the timber. The law here is a well defined one, so far as these timbers are concerned. 

 When rendered into English units (iwunds per sq. in.), the eciuation of this line is: 



Crushing strength = 13800 specific gravity — 900. (4) 



when the tiiiilier contains but 15 per ce7it of moisture. This equation would also vaiy in its con- 

 stants for each species of timber. 



Relation heticeen the compressire strength ami the pereentage of moisture. 



In Fig. 9 are plotted some very careful tests by Prof. Bauschinger to show the relation 

 between the percentage of moisture and the crushing strength. 



There is no question but the crushing and the shearing strength are both greatly reduced by 

 moisture. The crushing test also gives a very fair indication of the strength of the timber in all 

 other ways. In this instance four sticks were taken and sections tested first green, or having an 

 average of 37 per cent of moisture when coTiiputcd on the wet weight, or 59 per cent of moisture 

 when conq)uted on the diy weight, as is the practice in the tests made by this Depaitincnt. The 

 sticks were then dried until there was an average of 14.0 per cent moisture on the wet weight or 

 17 per cent of the dry weight. The remaining portions of the sticks were further syasoned until 

 there remained but 8.2 per cent moisture C((mputed on the wet weight, or 9 per cent moisture on 

 the dry weight, and then tested. This is a smaller percentage of moisture than out-door lumber 

 ever reaches, as the ordinary humidity of the external aii- will usually maintain at least 10 per 

 cent of moisture in all kinds of timber. 



When these three groups of results are plotted, and the most probable curve drawn tlirough 

 them, there is seen to be a remarkable increase in the crushing strength when the percentage of 

 moisture falls below fifteen or twenty. The variation in strength abt>ve that Ihnit is very small. 

 Prof. Bauschinger has published a great many such curves, all showing the same general law. 

 This curve illustrates the necessity for finding the percentage of moisture for every test of strength 

 made. 



Prof. Bauschinger has published very few tests showing the relations between the cross- 

 breaking strength and the moisture, but Fig. lOisa reproducticm of such results as he has given. 

 "When the percentage of moisture sinks as low as 10 there appears a wide variation t)f strength, 

 not satisfactorily explained. There w<mld seem to be a law of dependence, however, but less 

 marked than in the case of compressive strength. 



Relation hetireen specijie gravity and moisture. 



In Fig. 9 the "specific-gravity" curve vshows the relation between the specific gravity and the 

 percentage of moisture. At first the specific gravity diminishes rapidly as the percentage of 

 moisture is reduced, but when this has been reduced to 15 per cent the specific gravity changes 

 very little for any further reduction in moistuie. This shows that the shrinkage is insignificant 

 until the timber becomes nearly dry, when it swells and shrinks almost directly with the percent- 

 age of moisture, so that the weight of a unit volume, which is a measure of the sijccific gravity, 

 remains nearly constant. This curve is also only one of a great many similar ones given by I'rof. 

 Bauschinger. 



