26 OF ENGINEERING FORMULÆ.
extreme softness. The contraction of area at the
fracture forms an essential element in estimating
the quality.
2nd. The breaking strain of iron and puddled
steel plates is greater in the direction in which
they have been rolled than in the direction of their
breadth; but in cast steel the reverse.
3rd. Iron when fractured suddenly produces a
crystalline frachire; but if gradually, a fibrous
fracture. This accounts for the anomaly in the
supposed change of iron from a fibrous to a
crystalline character. Sudden shoulders which
prevent a regular elongation of fibre cause a
sudden snap.
4th. Strength of steel is reduced by being
hardened in water; but both its hardness and
toughness are increased by being hardened in oil.
Iron heated, and suddenly cooled in water, is
hardened, and the breaking strain (if gradually
applied) is increased, hut it is more likely to snap
suddenly. It is softened and its breaking strain
reduced if heated and allowed to cool gradually.
Iron if brought to a white heat is injured if it
be not at the same time hammered or rolled. Case-
hardening bolts weakens them.
5th. The shearing strain of steel rivets is one-
fourth less than their tensile strength. The
ordinary proportions of iron rivets are too small
when steel rivets are used for steel plates.
6th. The specific gravity is found to indicate
the quality pretty correctly.
7th. The experiments on iron give the following
breaking strains:—