It is beyond the province of these Notes to enter upon the general subject of the physical properties of materials. The meanings of a few of the terms used in connection with those properties are given at pp. 467-470, and the subject will be further entered upon in Part IV.

The value of iron and steel to the engineer is, however, so entirely dependent upon their strength, ductility, etc., that a few observations on these points will be necessary in order to clear the way for an intelligent selection and testing of these materials.

In considering the strength of materials care must be taken to distinguish between the ultimate strength - that is, the stress per square inch of section which will cause rupture - and the working strength, or the stress per square inch which the material can safely bear in practice.

In the following pages the ultimate strength, as found by experiment, will first be given for various descriptions of iron and steel.

The effect upon this strength, produced by various circumstances, will be briefly mentioned.

The working stresses that may be permitted in practice will then be stated.

Finally, one or two points regarding the effect of vibration, cold, etc., will be merely glanced at.