its temperature is below 0 deg. C. It is about
10 deg. C. I can place over the ice a roof-shaped cover made of two
inclined slabs of thick plate glass. This acts to keep out warm
air, and to do away with any possibility of the surface of the
ice being wet with water thawed from the ice. The whole tank
along with its roof of glass can be adjusted to any angle, and a,
scale at the

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raised end of the tank gives the angle of slope in degrees. A
weight placed on the ice can be easily seen through the glass
cover.

The weight we shall use consists of a very light ring of
aluminium wire which is rendered plainly visible by a ping-pong
ball attached above it. The weight rests now on a copper plate
provided for the purpose at the upper end of the tank. The plate
being in direct contact beneath with the freezing mixture we are
sure that the aluminium ring is no hotter than the ice. A light
jerk suffices to shake the weight on to the surface of the ice.

We find that this ring loaded with only the ping-pong ball, and
weighing a total of 2.55 grams does not slip at the low angles. I
have the surface of the ice at an angle of rather over 131/2, and
only by continuous tapping of the apparatus can it be induced to
slip down. This is a coefficient of 0.24, and compares with the
coefficient of hard and smooth solids on one another. I now
replace the empty ping-pong ball by a similar ball filled with
lead shot. The total weight is now 155 grams. You see the angle
of slipping has fallen to 7 deg..

Every one who has made friction experiments knows how
unsatisfactory and inconsistent they often are. We can only
discuss notable quantities and broad results, unless the most
conscientious care be taken to eliminate errors. The net result
here is that ice at about -10 deg. C. when pressed on by a very light
weight possesses a

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coefficient of friction comparable with the usual coefficients of
solids on solids, but when the pressure is increased, the
coefficient falls to about half this value.

The following table embodies some results obtained on the
friction of ice and glass, using the methods I have shown you. I
add some of the more carefully determined coefficients of other
observers.

Wt. in On Plate On Ice On Ice
Grams. Glass. at 0 deg. C. at 10 deg. C.

Angle. Coeff. Angle. Coeff. Angle. Coeff
Aluminium 2.55 121/2 deg. 0.