The strength and ductility of steel varies greatly in different descriptions. It depends not only upon the original composition of the metal, but also upon the treatment to which it has been subjected, especially the rate of cooling. The following Tables give some idea of the variety to be met with in different specimens.

Average Strength.. - The great differences in strength caused by varieties in the amount of carbon and in temper make it useless to attempt to arrive at an average strength for all steels.

The following may be taken as a low average for the ultimate strength of soft cast-steel which has not been hardened : -

Tons per square inch.

Tension ....

32

Compression ....

30

Shearing ....

24

Mr. Matheson says - "A tensile and compressional strength equal to a breaking strain of 30 or 40 tons, with a limit of elasticity of 15 to 20 tons, may be stated as the quality of the plates L and T sections which are now made for constructional purposes." . . . "Steel equal to a tensile strength of from 40 to 55 tons is made for special purposes, such as chain-links for suspension bridges." . . . " With steel of this kind it is most important to know the limit of elasticity."l

The following Tables, selected from different records of experiments, show the great variation that there is in the strength and ductility of different descriptions of steel: -