AND STRAIN ON THE ACTION OF PHYSICAL FORCES. 
121 
through which the experimenter was enabled to adjust the weights on the scale-pan 
attached to the wire to be tested. The scale-pan was suspended from the pulley, 
which was 6 inches from the bottom of the chamber, by a wire sufficiently strong to 
bear the weights employed, and this wire passed through a hole in the table only just 
large enough to allow of free motion. The doors and windows of the room were kept 
shut during the testing at the higher temperature, and the usual precautions were taken 
to avoid any risk of permanent set or any liability to change of elasticity from testing 
too soon after permanent extension. In about an hour after the first entrance of the 
steam into the air chamber the temperature of the air at the top and bottom was 
nearly, if not quite, at 100° C, but it was found necessary to allow the steam to enter 
for three or four hours before the wire to be experimented on and the comparison-wire 
had assumed a sufficiently stable resistance-ratio, and even after this time there 
would be slow and very minute variations of this ratio first in one direction and 
then in the opposite. Any errors, however, which would result from slow and minute 
variations were got rid of in the following manner :—Let cq, b 1 ; a. 2 , b. 2 , &c., be the 
apparent alterations produced by putting on and taking off the load several times 
in succession; then the true alterations due to the load will be very nearly 
C] + ct„ + 2b^ b^ J rbc, + 2etc, „ 
1 , -4-"» &c - 
The following experiment will show how accurately the measurements could be made 
even at the temperature of 100° C. 
Experiment XL V. 
An annealed iron wire, 7 feet in length and '067 centim. in diameter, was loaded 
and unloaded several times with a weight of 3 kilogs.; this weight was then removed 
and a rest of 48 hours allowed, when, on again testing with 3 kilogs., the recovery 
was found to be quite perfect. The wire was then heated to 100° C., and having been 
maintained at this temperature for several hours was again tested. Afterwards the 
I air chamber was suffered to cool down to the original temperature of 13° C. and after 
> a rest of 24 hours the elasticity was redetermined with the same load as before. 
MDCOCLXXXIII. 
R 
