720 
MR. FAIRBAIRN’S EXPERIMENTAL INQUIRY 
In the first columns of the experiments it will be observed that the force required 
to break the bars was 32*37 tons, with a mean stretch of 9 inches upon twenty bars. 
In the second column the mean of the elongations, with a strain of 25 tons, was only 
1*6 inch, whereas the ultimate breaking strain w^as 32*8 tons, evidently showing an 
increase instead of a diminution of strength from the previous strain of 25 tons, to 
which the bars had been respectively subjected. 
Experiments made in the testing-machine of Woolwich Dockyard to ascertain the 
effect upon iron-bolt staves or iron bars to a tensile strain. The following results 
show the strains required for each of four successive breakages of the same pieces of 
iron as in the first experiment, ifths of an inch diameter S C 
Experiment 2. 
Distinguishing 
mark. 
First breakage. 
Second breakage. 
Third breakage. 
Fourth breakage. 
Redueed 
from 1-37 
to 
Tons. 
Stretch in 
54 inches. 
Tons. 
Stretch in 
36 inches. 
Tons. 
Stretch in 
24 inches. 
Tons. 
Stretch in 
15 inches. 
in. 
in. 
in. 
in. 
in. 
A. 
33‘75 
9-125 
35-5 
2-00 
C. 
33-75 
9-250 
35-25 
-25 
37-00 
1-00 
38-75 
1-25 
E. 
32-5 
9-250 
34-75 
1-25 
F. 
33-23 
10-500 
33-50 
1-12 
37-25 
•62 
•40 
1-18 
G. 
32-75 
8-500 
35-00 
1-25 
37*5 
•41 
1-25 
H. 
33-75 
10-625 
36-23 
1-87 
I. 
33-50 
8-375 
34-50 
-62 
36-5 
1-50 
J. 
33-50 
9-250 
, 36-00 
•25 
36-75 
1-120 
41-75 
1-25 
L. 
32-25 
Defective 
36-50 
1*5 
37-75 
41-00 
•31 
1-25 
M. 
30-25 
Defective 
36-50 
•62 
37*75 
•06 
38-50 
•06 
1-25 
Mean 
32-92 
35-57 
37*21 
40-16 
1-24 
Mean per square 
inch 
1 23-94 
25-86 
27-06 
29-20 
•90 
The results of the above experiments are highly interesting, as they not only con- 
firm those previously made, but they indicate a progressive increase of strength, not- 
withstanding the reduced sectional area of the bars. These interesting facts are of 
considerable value, as they show that a severe tensile strain is not injurious to the 
bearing powers of wrought iron even when repeated to the extent of four times. In 
practice it may not be prudent to test bars and chains to their utmost limit of resist- 
ance ; it is however satisfactory to know that in cases of emergency those limits may 
be approached without incurring serious risk of injury to the ultimate strength of 
the material. 
It is further important to observe, that the elongations are not in proportion to the 
forces of extension ; thus in the bar F, experiment 2, the elongation of a bar 54 inches 
long with 33*25 tons, is 10*5 inches, giving an elongation per unit of weight and 
10*5 
length = 2 ^ 725 "^^ = *0058 ; whereas an additional weight of 2*25 tons produces an 
