442 
Strength of wet or green wood is much more sensitive to changes of 
speed than is dry wood. At the speed adopted for official tests a change 
in speed of 50% may ordinarily be allowed without causing a variation in 
strength of over 2%. 
2. Temperature.—Since wood is a more or less plastic substance it is 
sensitive to changes of temperature. Tiemann’s? experiments show that 
soaking certain species in water at normal temperature does not affect 
their strength. It appears, however, that warm water has a marked weak- 
ening effect. The extreme condition is when wood is made pliable by boil- 
ing. Some woods are no doubt more sensitive than others to the effect of 
temperature of the water in which they are immersed. In recent tests 
made in winter weather on red oak (Quercus Rubra) ties at Purdue Uui- 
versity, ties taken from the temperature of the storehouse (about 25° F.) 
were from 9 to 17 per cent stronger than those tested at the temperature 
of the laboratory (about 70° F.). Probably this marked difference in 
strength is to be found only in case of green or wet wood. The rupture 
work is not affected to the degree of the ultimate strength. Tlickory seems 
specially sensitive to change of temperature. It is concluded that the 
ordinary temperature variations of the air of a laboratory are not im- 
portant, but that the temperature of the storehouse may render it neces- 
sary to warm the wood. In fact, the effect of a given factor on the 
strength of timber, or difference of strength of two species, may at times 
be entirely masked by variations of temperature of timber at the time 
of test. 
3. Moisture.—The effect of moisture on the strength of wood has 
been thoroughly investigated by Tiemann. His material was small test 
pieces uniform in moisture content throughout the cross-section; and he 
determined the distinct ‘fiber saturation’ point, above which increased 
moisture content did not affect the strength of timber and below which 
there was an increase of strength. Previous experiments, yielding a con- 
tinuous moisture strength law, were apparently made with “case-hardened 
material.” 
* Bulletin 70. Forest Service, 1906. ‘Effect of Moisture on the Strength and 
Stiffness of Wood,” by H. D. ‘Tiemann. 
* Bulletin 70. Forest Service, 1906. “Effect of Moisture on the Strength and 
Stiffness fo Wood,” by H. D. Tiemann. 
Circular 108, Forest Service, 1907. “The Strength of Wood as Influenced by 
Moisture,”’ by H. D. Tiemann. 
