20 
The fiber stress at the elastic limit and the modulus of rupture 
were computed for the diameter and moment of inertia obtaining 
under the smaller loading point, the section being regarded as a 
true circle. The stiffness 
factor y 
was obtained 
from the moment of in- 
ertia (I) and the deflec- 
tion (d) at the center of 
the beam. These values 
for stiffness are compar- 
able for pieces of the 
same span only. The 
true modulus of elasticity, 
which is difficult to ob- 
tain accurately for a beam 
\vdth a varying moment 
of inertia, depends upon 
the cube of the span. 
This relation is not con- 
sidered in the " stiffness 
P 
factor" used, y-i, and this 
causes the values for the 
7-foot span to be approxi- 
mately ten times as large 
as for the 15-foot span. 
The work to maximum 
load was calculated from 
the area under the load- 
deflection curve, and is 
given in terms of unit 
volume. In the calcula- 
tions involving the unit 
stresses of the props, the 
top area of the prop was 
used. 
The volume of the test 
pieces was calculated by 
multiplying the average 
-Method used in testing mine props in compression. Qf the areag Q f feh@ ^ 
the butt, and the mean proportional between them, by the length. 
The weight per cubic foot was obtained from this estimated vol- 
ume, and is therefore given as "approximate." 
