The component parts of a rigid body adhere to each other with a force which is termed cohesion.

Elasticity is the resistance which a body opposes to a change of form.

Strength is the resistance which a body opposes to a permanent separation of its parts.

Elasticity and strength, according to the manner in which a force is exerted upon a body, are distinguished as tensile strength, or ab-solute resistance; transverse strength, or resistance to flexure; crushing strength, or resistance to compression; torsional strength, or resistance to torsion; and detrusive strength; or resistance to shearing.

The limit of stiffness is flexure, and the limit of strength or resist-ann- is fracture.

Resilience, or toughness of bodies, is strength and flexibility combined; hence any material or body which bears the greatest load, and bends the most at the time of fracture, is the toughest.

The specific gravity of iron is ascertained to indicate very correctly the relative degree of its strength.

The neutral axis, or line of equilibrium, is the line at which extension terminates and compression begins.

The resistance of cast iron to crushing and tensile strains is, as a mean, as 4, 3 to 1.*

English cast iron has a higher resistance to compression, and a less tensile resistance, than American.

The mean tensile strength of American cast iron, as determined by Major Wade for the U. S. Ordnance Corps, is 31,829 lbs. per square inch of section; the mean of English, as determined by Mr. E. Hodgkinson for the Railway Commission, etc., in 1MB, is 19,484 lbs.; and by Col. Wilmot at Woolwich, in 1858, for gun-metal, is 23,257 lbs.

The ultimate extension of cast iron is the 500th part of its length.

The mean traverse strength of American cast iron, also deter-mined by Major Wade, is 681 lbs. per square inch, suspended from a bar fixed at one end and loaded at the other: and the mean of English, as determined by Fairbairn, Barlow, and Others, is 500 lbs.

The resistance of wrought iron to crushing and tensile strains is, as a mean, as 1.5 to 1 for American: and for English, I.2 to 1.

The mean tensile strength of American wrought iron, as determined by Prof. Johnson, is 55,900 lbs.. and the mean of English, as determined by Capt. Brown, Barlow, Brunei, and Fairbairn, is 53,900 lbs.f

The ultimate extension of wrought iron is the 600th part of its length.

The resistance flexure, acting evenly over the surface, is nearly ½ the tensile resistance.