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380                       METALLURGY   OF   CAST   IRON.
vertically-poured castings, in the sense which has beeti generally accepted.
In considering the gun and gate tests of specific-gravity in connection with those referring to the density of the lower side of flat-cast test bars being greater than the top side, discussed in Chapter LXV., it would at first seem as if the results were contra-dictory as far as they relate to the enunciation of1 any law or principle governing the quality of specific gravity in vertical-poured casting. The gate and gun tests show the upper end to have the greater specific gravity, and that of flat poured test bars to have the greater density in the side cast downwards. The latter is largely due to the bottom portion. or surface of flat-cast test bars being most affected by the chilling qualities of the sand of the mould when it is filled with molten metal. If the specific gravity had been taken from the bottom surface of the gate test bar and gun castings, instead of a few inches in height from their bottom end, as was done, there might have been a difference found in favor of the lower end being the denser. This is, however, doubtful, as the gun and gate specimens had such a small area exposed to the mould's cooling influence, compared to the mass of metal comprising the castings. On the other hand, with test bars cast flat, the reverse occurred, and this is due to the fact that a fair percentage of the metal comprising the test bars is distributed over a large area of mould surface and is affected by the cooling qualities of damp sand, which is an unnatural effect that cannot be charged to specific gravity proper.
When the specific gravities of long vertical-pouredBuilders' Iron Foundry, from whom the author received these tests, and wishes here to tender his thanks for the kindness rendered.