If no external forces act upon a body its particles assume certain relative positions, and it has what is called its _natural shape and size_. If sufficient external force is applied the natural shape and size will be changed. This distortion or deformation of the material is known as the ~strain~. Every stress produces a corresponding strain, and within a certain limit (see _elastic limit_, in FUNDAMENTAL CONSIDERATIONS AND DEFINITIONS, above) the strain is directly proportional to the stress producing it.[1] The same intensity of stress, however, does not produce the same strain in different materials or in different qualities of the same material. No strain would be produced in a perfectly rigid body, but such is not known to exist. [Footnote 1: This is in accordance with the discovery made in 1678 by Robert Hooke, and is known as _Hooke's law_.] Stress is measured in pounds (or other unit of weight or force). A ~unit stress~ is the stress on a unit of the sectional { P } area. { Unit stress = --- } For instance, if a load (P) of one { A } hundred pounds is uniformly supported by a vertical post with a cross-sectional area (A) of ten square inches, the unit compressive stress is ten pounds per square inch. Strain is measured in inches (or other linear unit). A ~unit strain~ is the strain per unit of length. Thus if a post 10 inches long before compression is 9.9 inches long under the compressive stress, the total strain is 0.1 inch, and the unit l 0.1 strain is --- = ----- = 0.01 inch per inch of length. L 10 As the stress increases there is a corresponding increase in the strain. This ratio may be graphically shown by means of a diagram or curve plotted with the increments of load or stress as ordinates and the increments of strain as abscissæ. This is known as the ~stress-strain diagram~. Within the limit mentioned above the diagram is a straight line. (See Fig. 1.) If the results of similar experiments on different specimens are plotted to the same scales, the diagrams furnish a ready means for comparison. The greater the resistance a material offers to deformation the steeper or nearer the vertical axis will be the line. [Illustration: FIG. 1.--Stress-strain diagrams of two longleaf pine beams. E.L. = elastic limit. The areas of the triangles 0(EL)A and 0(EL)B represent the elastic resilience of the dry and green beams, respectively.]