I wish now to dwell upon that beautiful contrivance the modern
skate. It is a remarkable example of how an appliance can develop
towards perfection in the absence of a really intelligent
understanding of the principles underlying its development. For
what are the principles underlying the proper construction of the
skate? After what I have said, I think you will readily
understand. The object is to produce such a pressure under the
blade that the ice will melt. We wish to establish such a
pressure under the skate that even on a day when the ice is below
zero, its melting
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point is so reduced just under the edge of the skate that the ice
turns to water.
It is this melting of the ice under the skate which secures the
condition essential to skating. In the first place, the skate no
longer rests on a solid. It rests on a liquid. You are aware how
in cases where we want to reduce friction—say at the bearing of a
wheel or under a pivot—we introduce a liquid. Look at the
bearings of a steam engine. A continuous stream of oil is fed in
to interpose itself between the solid surfaces. I need not
illustrate so well-known a principle by experiment. Solid
friction disappears when the liquid intervenes. In its place we
substitute the lesser difficulty of shearing one layer of the
liquid over the other; and if we keep up the supply of oil the
work required to do this is not very different, no matter how
great we make the pressure upon the bearings. Compared with the
resistance of solid friction, the resistance of fluid friction is
trifling. Here under the skate the lubrication is perhaps the
most perfect which it is possible to conceive. J. Müller has
determined the coefficient by towing a skater holding on by a
spring balance. The coefficient is between 0.016 and 0.032. In
other words, the skater would run down an incline so little as 1
or 2 degrees; an inclination not perceivable by the eye. Now
observe that the larger of these coefficients is almost exactly
the same as that which Perry found in the case of well-greased
surfaces. But evidently no
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artificial system of lubrication could hope to equal that which
exists between the skate and the ice. For the lubrication here
is, as it were, automatic. In the machine if the lubricant gets
squeezed out there instantly ensues solid friction. Under the
skate this cannot happen for the squeezing out of the lubricant
is instantly followed by the formation of another film of water.
The conditions of pressure which may lead to solid friction in
the machine here automatically call the lubricant into
existence.
Just under the edge of the skate the pressure is enormous.
Consider that the whole weight of the skater is born upon a mere
knife edge. The skater alternately throws his whole weight upon
the edge of each skate. But not only is the weight thus
concentrated upon one edge, further concentration is secured in
the best skates by making the skate hollow-ground, _i.e._
increasing the keenness of the edge by making it less than a
right angle. Still greater pressure is obtained by diminishing
the length of that part of the blade which is in contact with the
ice. This is done by putting curvature on the blade or making it
what is called "hog-backed." You see that everything is done to
diminish the area in contact with the ice, and thus to increase
the pressure. The result is a very great compression of the ice
beneath the edge of the skate. Even in the very coldest weather
melting must take place to some extent.
As we observed before, the melting is instantaneous,
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Heat has not to travel from one point of the ice to another;
immediately the pressure comes on the ice it turns to water. It
takes the requisite heat from itself in order that the change of
state may be accomplished. So soon as the skate passes on, the
water resumes the solid state. It is probable that there is an
instantaneous escape, and re-freezing of some of the water from
beneath the skate, the skate instantly taking a fresh bearing and
melting more ice. The temperature of the water escaping from
beneath the skate, or left behind by it, immediately becomes what
it was before the skate pressed upon it.