The most intelligent engineers, after ascertaining by exhaustive physical tests what they need, present their "requirements" to the iron and steel makers, whose practical experience and science guide them in the protracted metallurgical experiments necessary to find the exact process required. The engineer verifies the product by further tests, and by practical use may find that his "requirement" needs further modifications. As a result of all this care, some degree of certainty is secured as to what the material may be expected to do. No doubt the chemical composition of the slag used at Avesta was known and met some equally well known want in the iron, and thus the result arrived at was one which had been definitely and intelligently sought. An important factor in selecting material for the cables of suspension bridges is its _true elastic limit_. By this term we mean the percentage of the total strength of the material which it can exert continuously without losing its resilience, i.e., its power to resume its former shape and position when stress is removed. Now, in the case particularly of steel wire as commonly furnished in spiral coils, the curve put into the wire in the process of manufacture seriously diminishes this available sustaining power. For it is evident that it would be unsafe to subject these cables at any time to a stress beyond their elastic limit. If, e.g., a snowstorm or a great crowd of people should load a bridge beyond this limit, when the extra weight was removed the cables could not bring the bridge back to its normal place, and the result would be a permanent flattening and weakening of the arch. By a process invented and patented by Col. Paine, the wire in the New York and Brooklyn bridge was furnished _straight_ instead of curved. Now, if a short piece of common steel wire is taken from the coil, and pulled toward a straight position, and then released, it springs back into its former curve; but if a short piece of the straight-furnished wire that was put into this bridge is bent, and then released, it springs back toward its straight position.