Experiments on wrought iron bars give a tensile strength of from 18,000 lbs. to 22,400 lbs. per square inch of its section, as just sufficient to balance the elasticity of the metal, and as a bar of it is extended the 10,000th part of its length for every ton of direct strain per square inch of its section, it is deduced that its elasticity is fully excited when it is extended the 1000th part of its length, and the extension of it at its limit of elasticity is estimated at the 1520th part of its length.

The mean tensile strength of wrought iron being from 55,000 to 65,000 lbs., the value of it, when subjected to a tensile strain, may be safely estimated at from ¼ to 1/3 or this, or of its breaking strain. A bar of wrought iron will expand or contract .000006614, or the 151,200th part of its length for each degree of heat; and assuming, as before stated for east iron, that the extreme range of temperature in the air in this country is 140°, it will contract or expand with this change .000926, or the 1080th of its length, which is equivalent to a force of 20,740 lbs. (9¼ tons) per square inch of section.

Experiments upon wrought iron, to determine the results from repeated heating and laminating, furnished the following:- From 1 to 6 reheating and rollings, the tensile strength increased from 43,904 lbs. to 61,824 lbs., and from 6 to 12 it was reduced to 43,904 again.

The tensile force of metals varies with their temperature, generally decreasing as the temperature is increased. In silver the tenacity decreases more rapidly than the temperature; in copper, gold, ana platinum it decreases less rapidly than the temperature.

In iron, the tensile strength at different temperature is as follows: 60°,1; 1140, 1.14; 212°, 1.2; 250°, 1.32; 270°, 1.35; 325°, 1.41; 435°, 1.4.