320 
PHYSICS: A. A. MICHELSON 
The formulae which follow are in fact sufficiently general to include 
every case thus far examined, including materials of widely different 
properties, such as lead, tin, copper, aluminum, zinc, iron, steel, quartz, 
glass, calcite, limestone, slate, marble, wax, pitch, gelatine and rubber. 
It may, however, be expected that a more thorough investigation will 
require modification in the formulae which may be made to fit special 
cases with greater accuracy. 
The type of strain selected for this investigation is the torsion of cy- 
lindrical rods, as this is the only strain in which the form remains unal- 
tered. It is very probable that the laws governing this special type may 
be made to include other distortions such as extension, compression, 
bending, etc. 
Very decided changes may be expected from the effects of tempera- 
ture and pressure,* but these may be taken into account by an appro- 
priate alteration in the value of the 'constants' which enter into the 
formulae. 
The apparatus employed for the investigation consisted in a light 
pulley with radius of 8 cm. over which passed two cords, the ends of 
which carried scale pans for holding weights. 
The specimen to be investigated had a diameter of 12 mm. at the ends 
while the intervening portion (75 mm. long) had a diameter of 4 mm. 
One end was clamped to the supporting frame and the other to the 
pulley which rests on a knife-edge in the axis. 
The tests consisted in measuring the angular position of the pulley 
by a micrometer at intervals of one minute while under a constant torque. 
Laws of Elastico-viscous Flow. — The behavior of any soHd under stress 
may be considered as the resultant of four elements: 
a. The elastic displacement, 
b. The elastico-viscous displacement, 
c. The viscous displacement, 
d. The lost motion. 
These will be considered in turn. 
The Elastic Displacement. — This is characterized by being approxi- 
* A preliminary investigation of the effect of hydrostatic pressure on elasticity and on 
viscosity was begun several years ago, which it was hoped would show results in conformity 
with those which maintain in the body of the earth — whose enormous pressure produces an 
increase in both rigidity and viscosity sufficient to make the body of the earth (which at its 
actual temperature under ordinary conditions would certainly be in a molten state) as solid 
as steel. This expectation has been partially realized for a number of materials, metallic 
and non-metallic; the results notwithstanding certain anomalies — traceable to the effects 
of previous history — showing a perceptible increase in rigidity and a very marked increase 
in viscosity even with the relatively small pressures obtainable in the laboratory. 
