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XX1.—Experiments to show how Failure under Stress occurs in Timber, its 
Cause, and Comparative Values of the Maximum Stresses induced when 
Timber is fractured in Various Ways. By Angus R. Fulton, B.Sc, 
A.M. Inst.C.E., Engineering Department, University College, Dundee. Com- 
municated by Professor W. Prppm. (With Eight Plates and Five Text 
Illustrations. ) 
(MS. received July 10, 1911. Read November 13, 1911. Issued separately August 30, 1912.) 
INTRODUCTION. 
The object of this investigation was not to obtain fresh data on the average 
streneth of timber when subjected to destructive stresses. This has been done 
in great detail by Bauscuincer, Lanza, JoHNnson, and others, and is still being 
carried on, in America, on a large scale under the Bureau of Forestry. The intention 
was rather (a) to find the effect of the medullary rays when timber was stressed in 
compression, tension, shear, or by cross-bending; and (b) to endeavour to connect 
up in a satisfactory way the maximum stresses induced in cross-breaking with 
those obtained by direct compression, tension, or shear. For this purpose, then, 
it was not necessary that the number of experiments should be excessive, but it 
was essential that the results obtained should be comparable one with another. 
To obtain this, a single tree of each variety of wood to be tested was procured, the 
test pieces were sawn out of the best part of the trunk, carefully stacked, and 
allowed to season for at least twelve months. 
As the timber was stored in a perfectly dry, well-ventilated room, as the 
specific gravity values were practically constant at the time of carrying out the 
tests, and as the tests were mainly relative ones, it was not thought necessary 
to determine the actual moisture conditions. 
Four varieties of wood were chosen—Oak, Pitch Pine, Ash, and Box. The two 
former are representative of the class in which the medullary rays are very pro- 
nounced, and the two latter, of those in which they are much less marked. 
The Oak specimens were cut from a trunk 21 inches diameter, the Ash 24 inches 
diameter, and the Box 8 inches diameter; and the place of growth was known only 
for the Oak, which was Scone Palace grounds. 
The test pieces were rectangular in section, with one pair of parallel faces as 
far as possible tangential to the annual rings, and the other pair consequently 
parallel to the medullary rays, the length being measured along the axis of the 
tree. The dimensions of the cross-section were approximately in the ratio of 2 
TRANS. ROY. SOC. EDIN., VOL. XLVIII., PART II. (NO. 21). 63 
