152 
continually with the time, the body is yiscous. If a tallow 
candle and a stick of sealing-wax be supported by the ends 
and slightly weighted in the middle, the candle, though a 
soft substance, will remain straight, while the sealing-wax 
will gradually bend more and more. Tallow is thus a soft or 
plastic solid ; sealing-wax a viscous fluid. Observe, that with 
the viscous fluid, it is time which is required to produce the 
efiect. 
Imagine a horizontal fixed plane, a short distance above 
which, a parallel fixed plane is made to move horizontally, 
and suppose the space between the two filled with some 
viscous fluid, as treacle, the stratum of the fluid in contact 
with the fixed plane will be at rest, the stratum in contact 
with the moving plane will be moving with the same 
velocity as the moving plane, and intermediate strata will move 
with velocities proportional to their distances from the fixed 
plane. If the application of force to the moving plane be not 
continued, it will soon be brought to rest by the viscosity of the 
fluid, and the measure of viscosity is the force necessary to 
maintain this velocity. The viscosity of a substance is measured 
then by the tangential force, or the unit area of either of two 
horizontal planes, unit distance apart, necessary to maintain 
a relative velocity of the planes of unit amount, the space 
between the planes being filled with the substance. 
This tangential force constitutes a shearing stress. Now, 
if ice be not allowed to melt, and the shearing stress be not 
enough to forcibly rupture the piece of ice, no motion at all 
takes place, that is, the ratio of force applied to velocity pro- 
duced is infinite, or ice is an infinitely viscous fluid, or an 
eminently rigid solid. We shall return to this subject of the 
force necessary to shear ice. 
Another consequence of the great increase of volume 
when water becomes ice is, that ice is lighter than water. It 
is important to bear in mind, that the masses of ice, which are 
