this manner, and
hence we cannot skate upon them. Only quite a few substances
expand on freezing, and it happens that their particular melting
temperatures or other properties render them unsuitable to
skating. The most abundant fluid substance on the earth, and the
most abundant substance of any one kind on its surface, thus
possesses the ideally correct and suitable properties for the art
of skating.

I have pointed out that the pressure must be such as to bring the
temperature of melting below that prevailing in the ice at the
time. We have seen also, that one atmosphere lowers the melting
point of ice by the 1/140 of a degree Centigrade; more exactly by
0.0075 deg.. Let us now assume that the skate is so far sunken in the
ice as to bear for a length of two inches, and for a width of
one-hundredth of an inch. The skater weighs,

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let us say--150 pounds. If this weight was borne on one square
inch, the pressure would be ten atmospheres. But the skater rests
his weight, in fact, upon an area of one-fiftieth of an inch. The
pressure is, therefore, fifty times as great. The ice is
subjected to a pressure of 500 atmospheres. This lowers the
melting point to -3.75 deg. C. Hence, on a day when the ice is at
this temperature, the skate will sink in the ice till the weight
of the skater is concentrated as we have assumed. His skate can
sink no further, for any lesser concentration of the pressure
will not bring the melting point below the prevailing
temperature. We can calculate the theoretical bite for any state
of the ice. If the ice is colder the bite will not be so deep. If
the temperature was twice as far below zero, then the area over
which the skater's weight will be distributed, when the skate has
penetrated its maximum depth, will be only half the former area,
and the pressure will be one thousand atmospheres.

An important consideration arises from the fact that under the
very extreme edge of the skate the pressure is indefinitely
great. For this involves that there will always be some bite,
however cold the ice may be. That is, the narrow strip of ice
which first receives the skater's weight must partially liquefy
however cold the ice.

It must have happened to many here to be on ice which was too
cold to skate on with comfort. The

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skater in this case speaks of the ice as too hard. In the
Engadine, the ice on the large lakes gets so cold that skaters
complain of this. On the rinks, which are