Barus and Strouhal— Viscosity of Steel. 31 
glass and for steel respectively and ¢ the corresponding tor- 
sion; and since &#,/H,=1/3 approximately, it appears that 
¢,=3¢, nearly. Hence for equal stresses, strains are thor- 
oughly unequal, and in the same measure are these compari- 
sons not thoroughly just. Of. xxxui, p. 30. 
2. A similarly important desideratum is the comparison of 
the viscosities of steel and of iron. In tables 29, 29A and 30 
such comparisons are made, though further results are not su- 
perfluous. It appears distinctly that during the first five or ten 
hours of detorsion the viscosity of iron is in a strikingly pro- 
nounced manner less than that of steel. As detorsion continues 
the viscosity of soft iron continually remains below that of 
steel, whereas the viscosity of drawn iron grows temporarily 
greater than steel but finally reaches the same value. In this 
case the motion-curve passes through an exceedingly sharp 
maximum, at which the viscous yielding of iron and of steel 
occurs at like rates. Having exhibited greater viscosity in 
drawn iron it finally, per circumflexion, merges ito a horizon- 
tal asymptote, which probably indicates subsidence of motion 
in each wire. ‘hese unexpected results show that the viscosity 
of steel is not uniformly greater than that of iron. Obviously 
glass-hard steel for the given stress is very much less viscous 
than iron. 
The sum of the twists is here nominally ¢,—(—¢,)=3860°. 
Particularly in the case of soft iron its efficient value is very 
decidedly less, however. The applied torsion carries the iron 
wire much beyond the limits of elasticity ; and so much of the 
stress vanishes instanter that the “after-action”” subsides within 
relatively small limits. The viscosity of (soft) iron is either 
less or greater than that of hard steel according as the applied 
torsional stress surpasses or falls below a certain critical value. 
This result again shows the importance of stress-measurement, 
as a means of coordinating the lingering of purely viscous de- 
formation and the instantaneous (?) deformation, 
In the above results the curvature of the (absolute) motion 
curve for a single wire increases uniformly from hard steel to 
soft steel; increases, moreover, from soft steel to iron. Curva- 
ture is greater in the glass curve than in hard steel curves. So 
far as we have observed (steel, iron, glass) viscous deformation 
occurs more nearly at uniform rates (linearly) in proportion as 
hardness is greater. Curvatures for early time are meant, the ~ 
later parts of curves being asymptotes. 
3. The remarks just made on the viscosities of glass and iron 
suggest this plausible inference: if glass and steel be alike sub- 
jected to a stated process of quenching, and if after the opera- 
tion has been performed glass be found to have retained a very 
high intensity of strain (Rupert’s drop), then, @ fortiori, steel, 
