190 
IOWA ACADEMY OP SCIENCE 
the conditions existing at rest. The test, however, proved quite definitely that 
the law of proportionality held, at least within a very small per cent. So, while 
what was looked for did not materialize, at least the method was valuable in 
settling the question. 
The indirect method is simpler in theory. If the above mentioned propor- 
tionality holds, the vibration of the pendulum, with the tested wire alone, 
should be simple harmonic, and the trace of the vibrating point if combined 
(at right angles) with uniform linear motion should give a sine curve. Photo- 
graphic paper was placed around the drum of a chronograph, and the vibrating 
spot of light traced its record on the revolving drum in a darkened room. 
The developed paper, before being measured, had to be corrected in two particu- 
lars. In the first place the spot of light moved along the tangent instead of 
along the arc of a circle. After this correction was made, the correction 
for the decay in the amplitude of vibration had to be made. This was done 
by determining the logarithmic decrement, and by it, correcting all the ordinates 
of the curve that were used. As a result of one hundred and twenty measure- 
ments, it was found that the vibration of the pendulum was simple harmonic, 
from the fact that within the limits of error of the above measurements and 
reductions, the curve representing the vibrations was a sine curve. Thus the 
two methods were in agreement, and as a result one feels more confidence con- 
cerning the assumption of the elastic action of the steel wire under torsion. 
