without hesitation: "Because the ice is so
smooth."

It was at once objected: "But you can skate on ice which is not
smooth."

This put the professor in a difficulty. Obviously it is not on
account of the smoothness of the ice. A piece of polished plate
glass is far smoother than a surface of ice after the latter is
cut up by a day's skating. Nevertheless, on the scratched and
torn ice-surface skating is still quite possible; on the smooth
plate glass we know we could not skate.

Some little time after this discussion, the connection between
skating and a somewhat abstruse fact in physical science occurred
to me. As the fact itself is one which has played a part in the
geological history of the earth,

[1] A lecture delivered before the Royal Dublin Society in 1905.

260

and a part of no little importance, the subject of skating,
whereby it is perhaps best brought home to every one, is
deserving of our careful attention. Let not, then, the title of
this lecture mislead the reader as to the importance of its
subject matter.

Before going on to the explanation of the wonderful freedom of
the skater's movements, I wish to verify what I have inferred as
to the great difference in the slipperiness of glass and the
slipperiness of ice. Here is a slab of polished glass. I can
raise it to any angle I please so that at length this brass
weight of 250 grams just slips down when started with a slight
shove. The angle is, as you see, about 121/2 degrees. I now
transfer the weight on to this large slab of ice which I first
rapidly dry with soft linen. Observe that the weight slips down
the surface of ice at a much lower angle. It is a very low angle
indeed: I read it as between 4 and 5 degrees. We see by this
experiment that there is a great difference between the
slipperiness of the two surfaces as measured by what is called
"the angle of friction." In this experiment, too, the glass
possesses by far the smoother surface although I have rubbed the
deeper rugosities out of the ice by smoothing it with a glass
surface. Notwithstanding this, its surface is spotted with small
cavities due to bubbles and imperfections. It is certain that if
the glass was equally rough, its angle of friction towards the
brass weight would be higher.

261

We have, however, another comparative experiment to carry out. I
made as you saw a determination of the angle at which this weight
of 250 grams just slipped on the ice. The lower surfa