. Public works . by reputable manufacturers in re- cent years. There is, therefore, small excuse for supply-ing other than dependable brass pipe nowadays. Experiments carried on by Inspector Jonson of theCatskiil aqueduct laboratory demonstrated that improp-erly cold-worked rods were in a state of initial stress,and that these stresses were frequently of important mag-nitude, so that in many cases a small or moderate incre-ment of stress, like temperature change or load, wouldproduce failure. By placing round rods in a lathe andremoving their exterior surface, accurate measurementsshowed that
. Public works . by reputable manufacturers in re- cent years. There is, therefore, small excuse for supply-ing other than dependable brass pipe nowadays. Experiments carried on by Inspector Jonson of theCatskiil aqueduct laboratory demonstrated that improp-erly cold-worked rods were in a state of initial stress,and that these stresses were frequently of important mag-nitude, so that in many cases a small or moderate incre-ment of stress, like temperature change or load, wouldproduce failure. By placing round rods in a lathe andremoving their exterior surface, accurate measurementsshowed that the length of the rod gradually increased,demonstrating that the interior metal had been undercompression and the exterior surface in tension. Con-versely, on boring a good sized core from the inside ofrods, the rods shortened, again demonstrating the samecondition. From the amount of expansion or contractionin length it was possible to calculate with close approxi-mation the stresses and strains in the A 72 X 48 X 48-INCH BRONZE SHAFT CAP. Martens and Heyn. the eminent German physicists,have developed a method for determining the tendencyof brass to crack by applying a solution of mercury saltto the surface of a brass rod under initial stress. Crackswould be developed almost instantaneously or in a fewminutes in some specimens, while in others the crackswould not become apparent for a number of days. Thismercury test has been used by the Board of Water Sup-ply and. while useful, has not proved infallible, somerods which gave negative results under this test subse-quently cracking. Moreover, it would be manifestly im-practicable to apply this test to the whole surface of everyrod of a large shipment, and a more practical test is de-sired by which one can make sure that brass rods or otherarticles being purchased or already in place are perma-nently free from the tendency to crack. After the discovery of the extensive cracking referredto, it was decided to use plain e
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