[bovey] presidential ADDRESS 21 



of the specimen. The maximum amount of extension, viz., -00708 inch, 

 occurred on the 1 Ith of May. The greatest observed rates of extension 

 and recovery per hour were respectively 7 and 8 one hundred thousandths 

 of an inch. On the 16th of May the load was reduced to 200 lbs., when 

 the amount of the extension was reduced to -0024 inch. One hour later 

 the reading had fallen to -00233 inch, but an increase in the humidity 

 then caused a corresponding increase in the extension of "00017 inch. 



When the specimen is saturated above the normal state, it becomes 

 very diflEicult, if not impossible, to obtain the coelHcient of elasticity with 

 any degree of accuracy, on account of the rapid evaporation which 

 necessarily takes place. All the experiments, however, indicate that the 

 presence of an excess of moisture, instead of being detrimental to the 

 tensile strength of the timber, really increases it. 



In approaching the ultimate tensile strength it is found that the rate 

 of yielding is much more rapid in proportion to the load, and the greatest 

 care must be exercised in detining the exact point of failure, as it is an 

 exceeding!}^ easy matter to make an error of even as much as 10 per 

 cent. In ever}^ case, as might be expected, the coefficient of elasticity is 

 raised by drying. 



Shearing Tests 



The experiments in shear show similar results to those in tension, 

 and emphasize even more definitely the fact that the sheainng strength 

 of a timber diminishes as it loses its moisture. This diminution of shear- 

 ing strength has, of course, an important bearing upon the transverse 

 strength, as in many cases it is found that a kiln-dried beam fails from 

 longitudinal shear much sooner than if the beam had been in a noi*mal 

 or even saturated state. 



Compression Tests. 



Probabl}' over 80 per cent of the timber used in construction is placed 

 so as to be subjected to compressive stress, and for this reason it is most 

 essential to determine the extent to which the compressive strength is 

 afî'ected as the timber passes from a kiln-dried condition to a condition of 

 complete saturation. The tests of kiln-dined specimens are valuable as 

 standards of reference, but as it is impossible to keep timber in a kiln- 

 dried state inasmuch as it gradually tends to pass into the normal state, 

 the tests made between these two conditions are of no practical value. 



The kiln-drying of timber increases the compressive strength often as 

 much as 50 to 75 per cent, and in some cases, the strength has even been 

 doubled. A large increase of strength might have been naturally 

 expected, as in the process of drying the walls of the cells are stitfened 

 and hardened and become more able to resist compressive force, but the 

 walls are also made more brittle and it is possible that a sudden blow 



