Fan, 26, 1871] 

which a railway wheel is subjected is more akin to a blow 
than a steady pull; and as, moreover, the pretended 
brittleness is attributed more to cast-iron than any other 
description of the metal, I have made yet another kind 
of experiment. I got a quantity of cast-iron garden nails, an 
inch and a quarter long and fin. thickin the middle. These 
I weighed, and selected such as were nearly of the same 
weight. I then arranged matters so that by removing 
a prop I could cause the blunt edge of a steel chisel, 
weighted to 4lb. 20z., to fall from a given height upon 
the middle of the nail as it was supported from each end, 
I,/sin. asunder. Inorder to secure ihe absolute fairness 
of the trials the nails were taken at random. and an ex- 
periment with a cold nail was always alternated with one 
at the ordinary temperature. The nails to be cooled were 
placed in a mixture of salt and snow, from which they 
were removed and struck with the hammer in less than 5°.” 
The collective result of the experiments, the details of 
which need not be given, was that 21 cold nails broke and 
20 warm ones. 
Dr. Joule adds, ‘‘ The experiments of Lavoisier and 
Laplace, of Smeaton, of Dulong and Petit, and of Trough- 
ton, conspire in giving a less expansion by heat to steel 
than iron, especially if the former is in an untempered 
state. Such specimens of steel-wire and of watch-spring 
as I possess expand less than iron. But this, as Sir W. 
Fairbairn observed to me, would in certain limits have the 
effect of strengthening rather than of weakening an iron 
wheel with a tire of steel. 
“The general conclusion is this: Frost does of make 
either iron (cast or wrought) or steel brittle, and that acci- 
dents arise from the neglect of the companies to submit 
wheels, axles, and all other parts of their rolling stock to 
a practical and sufficient test before using them.” 
Mr. Spence in his experiments decided on having some 
lengths of cast-iron made of a uniform thickness of $in. 
square, from the same metal and the same mould. 
He writes :—“‘ Two of the four castings I got seemed 
to be good ones, and I got the surface taken off, and made 
them as regular a thickness as was practicable. 
“‘T then fixed two knife-edged wedges upon the surface 
of a plank, at exactly nine inches distance from each other, 
with an opening in the plank in the intervening space, the 
bar being laid across the wedges, a knife-edged hook was 
hung in the middle of the suspended piece of the bar, to 
the hook was hung a large scale on which to place weights. 
“The bar was tried first at a temperature of 60° F. ; 
to find the breaking weight I placed 56lb. weights one 
after another on the scale, and when the ninth was put on 
the bar snapped. This was the only unsatisfactory experi- 
ment, as 14 or 28lb. might have done it, but I include it 
among the others. I now adopted another precaution, by 
placing the one end of the plank on a fixed point and the 
other end on toa screw-jack, by raising which I could, 
without any vibration, bring the weight to bear upon the 
bar. By this means, small weights up to 7lb. could be 
put on while hanging, but when these had to be taken off 
and a large weight put on, the scale was lowered to the 
rest, and again raised after the change was made. I may 
here state that a curious circumstance occurred twice, 
which seems to indicate that mere raising of the weight, 
without the slightest apparent vibration, was equal in effect 
to an additional weight. 3 }cwts. were on the scale,a 14lb. 
weight was added, then 7]b., then 4lb, 2lb., 1lb., and 1]b., 
making 4cwts. and 1lb. This was allowed to act for from 
one to two minutes, and then lowered to take off the small 
weights, and replaced by a 56lb., intending to add small 
weights when suspended, raised so imperceptibly by the 
screw, that the only way of ascertaining that it was 
suspended was by looking under the scale to see that it 
was clear of the rest. As soon asit was half-an-inch clear 
it snapped, thus breaking at once with one pound less than 
it resisted for nearly two minutes. 
“Six experiments were carefully conducted at 60° F., 
NATURE 2 

57 

the parts of the bars being selected so as to give to each 
set of experiments similar portions of both bars; the 
results are marked on the pieces. My assistant now pre- 
pared a refrigerating mixture which stood at zero, and the 
bars were immersed for some time in this, and we prepared 
for the breaking trials to be made as quickly as could be, 
consistently with accuracy, and to secure the low tempera- 
ture each bar on being placed in the machine had its sur- 
face at top covered with the freezing mixture. Zhe bars 
at zero broke with more regularity than at 60°, but instead 
of the results confirming the general impression as to cold 
rendering tron more brittle, they are calculated to substan- 
tate an exactly opposite idea, namely, that reduction of 
temperature, ceteris paribus, tzcreases the strength of cast 
zron. The only doubtful experiment of the whole twelve 
is the first, and as it stands much the highest, the proba- 
bility is that it should be lower ; yet, even taking it as it 
stands, the average of the six experiments at 60° F. gives 
4cwt. 4lb. as the breaking weight of the bar at that tem- 
perature, while the average of the six experiments at zero 
gives 4cwt. 2olb. as the breaking weight of the bar at zero, 
being an increase of strength from the reduction of tem- 
perature equal to 3°5 per cent.” 
Sir W. Fairbairn’s evidence is of great importance, for 
he not only gives facts showing that frost does not affect 
the tires, but he states the real cause of such accidents as 
are generally attributed to the frost. 
He states :—“ It has been asserted in evidence given at 
the coroner's inquest on the Hatfield accident, that the 
breaking of the steel tire was occasioned by the intensity 
of the frost, which is supposed to render the metal brittle, 
and of which this particular tire was composed. This is 
the opinion of most persons, but judging from my own 
experience such is not the fact, and provided we are to 
depend on actual experiment, it would appear that tem- 
perature has little or nothing to do with it. Some years 
since I endeavoured to settle this question by a long and 
careful series of experiments on wrought-iron, from which 
it was proved that the resistance to atensile chain was as 
great at the temperature of zero as it was at 60° or up- 
wards, until it attained a scarcely visible red heat. To 
show that this was the case, and taking, for example, the 
experiments at 60°, it will be found that the mean break- 
ing weight, in tons, per square inch, was in the ratio of 
19°930 to 21°879, or as I : 1'098 in favour of the speci- 
mens broken at the temperature of zero. The generally 
received opinion is, however, against these facts, and it is 
roundly asserted that the strength of iron and steel is 
greatly reduced in strength at a temperature below freezing, 
The contrary was proved to be the case in wrought-iron 
plates, and assuming that steel follows the same law, it 
appears evident that we must look for some other cause 
than change of temperature for the late fracture of the 
tire on the wheel of the break-van of the Great Northern 
Railway. - 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 know- 
ledge of their security under the various influences of 
those changes; and when it is known that most of our 
metal constructions are exposed to a range of tempera- 
tures varying from the extreme cold of winter to the 
intense heat of summer, it is assuredly desirable to ascer- 
tain the effects produced by those causes on material from 
which we derive so many benefits, and on the security of 
which the safety of the public frequently depends. It 
was for these reasons that the experiments in question 
were undertaken, and the summary of results are suffi- 
ciently conclusive to show that changes of temperature 
are not always the cause of failure, as that which occurred 
near Hatfield on the Great Northern Railway. That such 
is the fact, I may adduce several accidents of broken tires 
all of which occurred during the spring and summer 
