312 DE. FAIEBAIEN ON THE EEEECT OF IMPACT, YIBEATOEY ACTION, 
Few facts are known, and very few experiments have been made bearing directly on the 
solution of this question. It has been assumed, probably not without reason, that wrought 
iron of the best and toughest quality assumes a crystalline structure when subjected to 
long and continuous vibration — that its cohesive powers are much deteriorated, and it 
becomes brittle, and liable to break with a force considerably less than that to which 
it had been previously subjected. This is not improbable ; but we are apparently yet 
ignorant of the causes of this change, and the precise conditions under which it occurs. 
In the year 1837 I instituted a long series of experiments to determine an important 
quality in the strength of materials, viz. the powers of crystalline bodies to sustain 
pressure for an indefinite period of time, and to ascertain whether cast iron, when sub- 
jected by a given weight to long-continued transverse strain, would or would not be 
subject to fracture. 
It appears that former writers on the transverse strength of materials had come to 
the conclusion that the bearing-powers of cast iron were confined within the limits of 
that force which would produce a permanent set, and that it would be unsafe to load 
this material with more than one-third of the weight necessary to break it. This 
assumption is incorrect, as in the experiments to which we refer some of the bars, six 
in number, were loaded within one-tenth of the weight that would break them. 
From these experiments it was ascertained that cast iron, when sound, is more to be 
depended upon, and exhibits greater tenacity in resisting long-continued heavy strains, 
than is generally admitted, and its bearing-powers have deserved a much higher repu- 
tation than has at any former period been given to them. This is even more apparent 
with wrought iron, as it is safer, being more tenacious and ductile, and less liable to 
flaws and imperfections, which, too, should they exist, are much more easily detected 
than in cast iron. 
The experiments, as respects the effects of time, on loaded cast-iron bars 1 inch 
square and 4 feet 6 inches between the supports, were exceedingly curious and interest- 
ing. They embraced a period of seven years, from 1837 to 1844, when they were 
discontinued, — the heaviest-loaded bars continuing to sustain their load without any 
apparent increase in the deflection. The deflections were taken monthly and carefully 
recorded, and the following Table exhibits the changes that took place in both the hot- 
and cold-blast iron bars from June 1838 to June 1842. It is satisfactory to observe 
that during the whole time of the experiments the bars, whether loaded with the 
lighter or heavier weights, exhibited little or no change beyond what may be traced to 
the variations of temperature. One of the bars was, however, found broken, but 
whether from accident or the effects of continued strain I am unable to determine. I 
am inclined to believe that the former was the case, as the corresponding bars retained 
their position, indicating changes so exceedingly small as to be scarcely perceptible, 
even when examined by the microscope and our best instruments. 
