322 DR. FAIRBAIRN ON THE EFFECT OF IMPACT, VIBRATORY ACTION, 
At this point, the beam having sustained upwards of three million changes of load 
without any increase of the permanent set, it was assumed that it might have con- 
tinued to bear alternate changes to any extent with the same tenacity of purpose as 
exhibited in the foregoing Table. It was then concluded to increase the load from one- 
fourth to one-third of the breaking-weight ; and having laid on 4 tons, which increased 
the deflection to *20 inch, the work was proceeded with in the same order as in the 
previous experiments. 
Table IV. 
Date. 
Changes of load. 
Deflection, in 
inches. 
Permanent set, 
in inches. 
Remarks. 
1861. 
Oct. 18. 
0 
0-20 
19. 
4000 
0-20 
0* 
Nov. 18. 
126,000 
0-20 
Dec. 18. 
237,000 
0-20 
1862. 
Jan. 9. 
313,000 
Broke by tension across the bottom web. 
From these experiments it is evident that wrought-iron girders of ordinary con- 
struction are not safe when submitted to violent disturbances with a load equivalent to 
one-third the weight that would break them. They, however, exhibit wonderful 
tenacity when subjected to the same treatment with one-fourth the load; and assuming 
that an iron girder bridge will bear with this load 12,000,000 changes without injury, 
it is clear that it would require 328 years, at the rate of 100 changes per day, before its 
security was aflected. It would, however, be dangerous to risk a load of one-third the 
breaking-weight upon bridges of this description, as according to the last experiments 
the beam broke with 313,000 changes; or a period of eight years, at the same rate as 
before, would be sufficient to break it. It is more than probable that the beam might 
have been injured by the previous three million changes to which it had been subjected ; 
and assuming this to be true, it would then follow that the beam was progressing to 
destruction, and must of necessity at some time, however remote, have terminated in 
fracture. 
The experiments throw considerable light on this very intricate and very important 
subject. They are probably carried sufficiently far to enable us to state with certainty 
what is the safe measure of strength ; and as much depends upon the quality of the 
material and the skill with which the girders are put together, it becomes necessary 
for the public safety that a measure of strength should be established without encum- 
bering the structures with unnecessary weight. On this question it must be borne 
in mind that every additional ton that is not required beyond the limits of safety, is 
an evil that operates as a constant quantity tending to produce rupture; and hence 
follows the necessity of a careful distribution of the material, in order that the tube or 
girder shall be duly proportioned to the strains it has to bear, and that every part of the 
structure shall have its due proportion of work to perform. 
