These definitions being understood, a brief description of the heat treatment can be taken up, because it is after this treatment that standard bars are taken from the forgings to undergo the physical tests. The first step consists in “tempering” or hardening the metal. The piece to be tempered is placed in an upright position in a high furnace and uniformly heated to the required temperature. It is then lifted from the furnace through an opening in the top and carried by a crane to an oil tank of suitable depth and plunged into the oil. This rapid cooling or “tempering in oil” is facilitated by having the oil tank surrounded by a water bath, so arranged that a supply of cold water is constantly in circulation to carry the heat from the mass as quickly as possible. This operation produces exceeding toughness, increases the tensile strength and raises the elastic limit of the metal. Now the forging is again annealed, so as to relieve any strains set up by tempering and to soften up the metal to the degree required by the specifications. It also increases materially the elongation and contraction. Great care must be exercised in the heat treatment, as the acceptance or rejection of the forging depends upon whether or not the test bars pass the required specifications. The forging is now submitted for test and the test bars taken. In the manufacture of a big gun, four test bars are taken from the breech end and four from the muzzle end of each forging and these bars sent to the physical laboratory. Quite an elaborate testing machine is provided, and if the bars pass the required tests the forging is accepted and is sent to the machine shop for finish-boring and turning. ~SEARCHING FOR POSSIBLE DEFECTS~ Frequently during finish-boring the work is examined to see that the bit is running true, and great care must be exercised to prevent its running out of alignment. After finish-boring every forging is “bore-searched,” that is, the bore is carefully examined for any cracks, flaws, streaks or discoloration. A special instrument called a “bore-searcher” is used and consists of a long wooden handle which has a mirror inclined at 45° at one end, together with a light to illuminate the bore, and so shielded as to obscure the light from the observer. (See sketch.) [Illustration] The bore is also inspected by the foreman after each boring, but the final “bore-searching” is done by an inspector. Now to measure accurately the inside diameters of long cylinders, such as are used in gun work, a special measuring device called a “star-gauge” is used. Its name is derived from the fact that it has three measuring points set at 120° apart and two measurements are taken, one [Illustration] and the other [Illustration], making a star [Illustration]. Every forging is “star-gauged” after being finish-bored and also the liner of the _gun_ after each assemblage operation. ~PUTTING THE PARTS OF A “BUILT-UP” GUN TOGETHER~