422 REPORT — 1856. 



From the experiments on rivet iron we have a mean elongation, in four- 



•643 

 teen experiments, of "643 inches in 2^ inches, or ^27=- = "257 per unit of 



length ; and in those on the S. C. — bars, we have a mean elongation of 

 •274, as given in the following Table : — 



Hence it appears that the rate of elongation of bars of wrought iron in- 

 creases with the decrease of their length ; thus while a bar of 120 inches has 

 an elongation of '216 inch per unit of its length, a bar of 10 inches has an 

 elongation of "42 per unit of its length, or nearly double what it is in the 

 former case. The relation between the length of the bar and its maximum 

 elongation per unit, may be approximately expressed by the following 

 formula, viz. — 



2-5 

 /=-18 + -f-, 



where L represents the length of the bar, and / the elongation per unit of 

 length of the bar. 



It is difficult to measure accurately the elongations in 2^ inches, but the 

 following Table shows the elongation per unit of weight and length at 

 various temperatures, as exhibited in the experiments on rivet iron. 



The two first experiments, at low temperatures, are rather anomalous, but 

 the rest are more consistent, showing that the elongation per unit of length 

 and weight is nearly the same at all ordinary temperatures, but is more than 

 doubled at red heat. 



