Two documents published some time since illustrate the preceding remarks. The first is the account of the tests of the iron taken from the Tariffville bridge after its failure, and the second is the specification for bridges on the Cincinnati Southern Railroad. The Tariffville bridge, though nominally a wooden one, like most structures of the kind relied entirely upon iron rods to keep the wood-work together. Although the rods were too small, and seriously defective in manufacture, the bridge ought not to have fallen from that cause. The ultimate strength of the iron was not what it should have been, but yet it was not low enough to explain the disaster; but when we look at the _quality_ of the iron, we have the cause of the fall. The rods taken from the bridge show an ultimate tensile strength of 47,560 pounds per inch, but an elastic limit of only 19,000 pounds; while the strain which was at any time liable to come on them was 22,000 pounds per inch, or 3,000 pounds more than the elastic limit. The fracture of the tested rods, which, it is stated, broke with a single blow of the hammer very much in the manner of cast-iron, showed a very inferior quality of metal. The rods broke in the bridge exactly where we should look for the failure; viz., in the screw at the end. No ordinary inspection would have detected this weakness. No inspection _did_ detect it, but a proper specification faithfully carried out would have prevented the disaster. Look now at an extract from the specification for bridges upon the Cincinnati Southern Railway:--