420 



REPORT — 185fi. 



Table XXX.— Sectional area ='24850. 



Tempera tui'B raised to red heat, visible by daylight. 



Broke with the weight of the lever =8,965 lbs. 



Elongation =^'55. 



Breaking weight per square inch =36,076 lbs. =16'105 tons. 



In this experiment, as in those on the plates, the tenacity of the iron is 

 seriously injured before the temperature reaches dull red heat ; and when that 

 point is attained, it has lost more than one-half its powers of resistance to 

 strain. At this high temperature it becomes exceedingly ductile and weak 

 when subjected to any description of force, inasmuch as it becomes so pliable 

 that it is immaterial whether the strain applied is compressive, tensile or 

 torsional. Under any of these forces it is not to be depended upon at a 

 temperature bordering upon redness. 



Collecting the results of the foregoing experiments in their consecutive 

 order into a Table, we see that the maximum strength of bars appears to be 

 attained at a mean temperature of about 320°. This is above the tempera- 

 ture at which the maximum strength of the plates was attained ; but it is to 

 be remembered, that little or no change is observable in the strength of the 

 plates, whilst that of the bars is increased nearly one-half. 



This fact is worthy of notice, inasmuch as in countries where the climate 

 is hot and never descends below freezing, the best bar iron will retain a 

 power of resistance equal to 29 tons upon the square inch, whereas in colder 

 and more northerly districts it would not be safe to calculate upon more 

 than 28 tons to the square inch. 



General Summary of Results. 



In the above Table we perceive a steady improvement in the strength of 

 the iron from 60° up to 325°, where the maximum appears to be attained. 

 As already noticed, this improvement does not present itself in the inferior 

 descriptions of irons, such as the plates tested in the preceding experiments. 

 This may arise from the different processes pursued in the manufacture, the 

 bars being rendered fibrous and ductile, in the first instance, under the 

 hammer, and this is further improved by reheating them and passing them 

 between the rolls. Bar iron will thus be drawn by the hammer and rolls- 1 



