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.

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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.

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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 surface of the weight, the part which presses on the ice, consists of a light, brass curtain ring.  This can be detached.  Its mass is only 61/2 grams, the curtain ring being, in fact, hollow and made of very thin metal.  We have, therefore, in it a very small weight which presents exactly the same surface beneath as did the weight of 250 grams.  You see, now, that this light weight will not slip on ice at 5 or 6 degrees of slope, but first does so at about io degrees.

This is a very important experiment as regards our present inquiry.  Ice appears to possess more than one angle of friction according as a heavy or a light weight is used to press upon it.  We will make the same experiment with the plate of glass.  You see that there is little or no difference in the angle of friction of brass on glass when we press the surfaces together with a heavy or with a light weight.  The light weight requires the same slope of 121/2 degrees to make it slip.