ntity of a liquid which
wets glass be introduced between two glass plates slightly inclined to
each other, it will run towards that part where the glass plates are
nearest together. When the liquid is in equilibrium it forms a thin
film, the outer edge of which is all of the same thickness. If d is the
distance between the plates at the edge of the film and [PI] the
atmospheric pressure, the pressure of the liquid in the film is [Pi] -
(2T cos[alpha])/d, and if A is the area of the film between the plates
and B its circumference, the plates will be pressed together with a
force

2AT cos[alpha]
------------- + BT sin[alpha],
d

and this, whether the atmosphere exerts any pressure or not. The force
thus produced by the introduction of a drop of water between two plates
is enormous, and is often sufficient to press certain parts of the
plates together so powerfully as to bruise them or break them. When two
blocks of ice are placed loosely together so that the superfluous water
which melts from them may drain away, the remaining water draws the
blocks together with a force sufficient to cause the blocks to adhere by
the process called _Regelation_.

[An effect of an opposite character may be observed when the fluid is
mercury in place of water. When two pieces of flat glass are pressed
together under mercury with moderate force they cohere, the mercury
leaving the narrow crevasses, even although the alternative is a vacuum.
The course of events is more easily followed if one of the pieces of
glass constitutes the bottom, or a side, of the vessel containing the
mercury.]

In many experiments bodies are floated on the surface of water in order
that they may be free to move under the action of slight horizontal
forces. Thus Sir Isaac Newton placed a magnet in a floating vessel and a
piece of iron in another in order to observe their mutual action, and
A.M. Ampere floated a voltaic battery with a coil of wire in its circuit
in order to observe the effects of the earth's magnetism on the electric
circuit. When such floating bodies come near the edge of the vessel they
are drawn up to it, and are apt to stick fast to it. There are two ways
of avoiding this inconvenience. One is to grease the float round its
water-line so that the water is depressed round it. This, however, often
produces a worse disturbing effect, because a thin film of grease
spreads over the water and increases its surface-viscosity. The other