Fig. 1438.
Suppose in [Fig. 1438] a a represents a piece of wrought-iron tube that has been heated to a bright red and immersed in cold water c c from the end b to d, until that end is cold. The part submerged and cold will be contracted to its normal diameter and have regained its normal strength, while the part above the water, remaining red-hot, will be expanded and weak. There will be, then, a narrow section of the tube, joining the heated and expanded part to the cooled and contracted part, and its form will be conical, as shown at d d. Now, suppose the tube to be slowly lowered in the water, the cold metal below will compress the heated metal immediately above the water-line, the cone section d being carried up into the metal before it has had time to cool; and the tube removed from the water when cold will be as shown in [Fig. 1438], from c to d, representing the part first immersed and cooled. To complete the operation the tube must be heated again from the end c to a short distance past d, and then immersed from e nearly to d, and held still until the submerged part is cold, when the tube must be slowly lowered to compress the end c d, making the tube parallel, but smaller in diameter and in bore, while leaving it of its original length, but thickening its wall.
Fig. 1439.
This process may, in many cases, be artificially assisted. Suppose, for example, a washer is too large in its bore; it should have its hole and part of its radial faces filled with fire-clay, as shown in [Fig. 1439], in which a is the washer and b b the clay, c c being pieces of wire to hold the fire-clay and prevent its falling off. The washer should be heated to a clear red and plunged in the water d d, which will cool and shrink the exterior and exposed metal in advance of the interior, which will compress to accommodate the contraction of the outer metal, hence the hole will be reduced. This operation may be repeated until the hole be entirely closed.
Fig. 1440.