ges 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, and to the hook was hung a large scale on which to place
weights.

"The bar was tried first at a temperature of 60 deg. 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 experiment, as 14 or 28lb. might have done it, but I
include it among others. I now adopted another precaution, by placing
the one end of the plank on a fixed point and the other end on to a
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. 33/4 cwts. were on the scale, a 14lb.
weight was added, then 7lb., then 4lb., 2lb., 1lb., and 1lb., making
4cwts. and 1lb. This was allowed to act for from one to two minutes,
and then lowered to take off the small weights, which were replaced by
a 56lb. with the intention of adding small weights when suspended; the
whole was then 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 as it 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 deg. F., 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 prepared a refrigerating mixture which stood at zero,
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 temperature, each bar, on being placed
in the machine, had its surface at top covered with the freezing
mixture. The bars at zero broke wi