412 



REPORT — 1856. 



inches. However, the elongation of these short specimens cannot always be 

 depended on, as there is considerable difficulty in ascertaining them accu- 

 rately. 



Table XV. — Strain applied across the fibre. 

 Boiler plate ; sectional area =2*0 x "31 ="62 sq. inch. 



The plate in this experiment was heated until it became perceptibly lumi- 

 nous in the shade ; it was then loaded, as before, until fracture ensued. In 

 this experiment it will be observed that a considerable diminution of strength 

 took place in consequence of the increased temperature, clearly showing that 

 above a certain point the tensile strength of wrought iron is seriously injured. 

 This fact is more strikingly apparent in the next experiment, in which the 

 temperature was raised to a dull red heat, just perceptible in daylight. 



Table XVI. 



In this experiment a plate of the same description as the last was raised 

 to a dull red heat, when the weight of the lever was allowed to strain the 

 specimen with a force of ISjSiOlbs., and fracture immediately ensued. The 

 elongation was -23. 



Sectional area of boiler plate=l* 96 X '31 ='6076 sq. inch. 



Strain applied across the fibre. 



Breaking weight per square inch =30,5131bs.= 13"621 tons. 



This experiment is quite conclusive as to the effects produced on wrought 

 iron whenever it approaches a red heat. At that temperature nearly one- 

 half its strength is lost ; it becomes exceedingly ductile, and is drawn con- 

 siderably in the direction of the strain before its cohesive powers are 

 destroyed. 



The greatly increased ductility of wrought-iron plates, at a dull red heat, 

 is strikingly exemplified in the flues of boilers, whenever the water gets low, 

 or recedes below the surface of the plates, and that more particularly if the 

 plates are immediately over the fire; in such a position the flues readily collapse 

 with a comparatively low pressure. In the bending of a plate, when red hot, 

 a very small force is required ; but within limits of temperature not exceed- 

 ing 400°, it requires nearly the same force to produce collapse as it would 

 at any temperature above 32°, or the freezing-point of water*. 



* We hope in a short time to give a series of experiments on the resistance of wrought- 

 iron plates and bars to a transverse and compressive force at various temperatures. 



