th more regularity than at 60 deg., but
instead of the results confirming the general impression as to cold
rendering iron more brittle, they are calculated to substantiate
an exactly opposite idea, namely, that reduction of temperature,
_caeteris paribus_, increases the strength of cast iron. The only
doubtful experiment of the whole twelve is the first, and as it stands
much the highest, the probability is that it should be lower; yet,
even taking it as it stands, the average of the six experiments at
60 deg. F., gives 4cwt. 4lb. as the breaking weight of the bar at that
temperature, while the average of the six experiments at zero gives
4cwt 20lb. as the breaking weight of the bar at zero, being an
increase of strength, from the reduction of temperature, equal to 3.5
per cent."

Sir W. Fairbairn states: "It has been asserted, in evidence given at
the coroner's inquest, in a recent railway accident, that the breaking
of the steel tire was occasioned by the intensity of the frost, which
is supposed to have rendered the metal, of which this particular
tire was composed, brittle. This is the opinion of most persons, but
judging from my own experience such is not the fact. Some years since
I endeavored to settle this question by a long and careful series
of experiments on wrought iron, from which it was proved that the
resistance to a tensile chain was as great at the temperature of zero
as it was at 60 deg. or upwards, until it attained a scarcely visible red
heat."

The immense number of purposes to which both iron and steel are
applied, and the changes of temperature to which they are exposed,
renders the inquiry not only interesting in a scientific point of
view, but absolutely necessary to a knowledge of their security under
the various influences of those changes. It was for these reasons
that the experiments in question were undertaken, and the summary of
results is sufficiently conclusive to show that changes of temperature
are not always the cause of failure. Sir W. Fairbairn adds: "The
danger arising from broken tires does not, according to my opinion,
arise so much from changes of temperature as from the practice of
heating them to a dull red heat, and shrinking them on to the rim of
the wheels. This, I believe, is the general practice, and the unequal,
and in some cases, the severe strains to which they are subject, has a
direct tendency to break the tires."

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