chiefly used there, the
ice is frequently renewed by flooding with water at the close of
the day. It thus never gets so very cold as on the lakes. I have
been on ice in North France, which, in the early morning, was too
hard to afford sufficient bite for comfort. The cause of this is
easily understood from what we have been considering.

We may now return to the experimental results which we obtained
early in the lecture. The heavy weights slip off the ice at a low
angle because just at the points of contact with the ice the
latter melts, and they, in fact, slip not on ice but on water.
The light weights on cold, dry ice do not lower the melting point
below the temperature of the ice, _i.e._ below -10 deg. C., and so
they slip on dry ice. They therefore give us the true coefficient
of friction of metal on ice.

This subject has, more recently been investigated by H. Morphy,
of Trinity College, Dublin. The refinement of a closed vessel at
uniform temperature, in which the ice is formed and the
experiment carried out, is introduced. Thermocouples give the
temperatures, not only of the ice but of the aluminium sleigh
which slips upon it under various loads. In this way we may be
certain that the metal runners are truly at the temperature of
the ice. I now quote from Morphy's paper

281

"The angle of friction was found to remain constant until a
certain stage of the loading, when it suddenly fell to about half
of its original value. It then remained constant for further
increases in the load.

"These results, which confirmed those obtained previously with
less satisfactory apparatus, are shown in the table below. In the
first column is shown the load, _i.e._ the weight of sleigh +
weight of shot added. In the second and third columns are shown,
respectively, the coefficient and angle of friction, whilst the
fourth gives the temperature of the ice as determined from the
galvanometer deflexions.

Load. Tan y. y. Temp.

5.68 grams. 0.36+-.01 20 deg.+-30' -5.65 deg. C.
10.39 -5.65 deg.
11.96 -5.75 deg.
12.74 -5.60 deg.
13.53 -5.65 deg.
14.31 -5.65 deg.
15.10 grams. 0.17+-.01 9 deg..30'+-30' -5.60 deg.
16.67 -5.55 deg.
19.81 -5.60 deg.
24.52