[Illustration:  FIG. 1.]

[Illustration:  FIG. 2.]

Beginning with the raw material, we were shown a disk of metal like that shown in Fig. 1, and measuring thirty inches in diameter and three-quarters of an inch in thickness.  From such a “blank” a cylinder destined to hold 100 feet of compressed gas can be constructed, and the first operation is to heat the “blank” in a furnace, and afterward to stamp it into the cup-like form shown in Fig. 2.  To all intents and purposes this represents the end of a finished cylinder, but it is far too bulky to form the end of one of the size indicated; indeed, it in reality contains enough metal to make the entire vessel.  By a series of operations it is now heated and drawn out longer and longer, while its thickness diminishes and its diameter grows less.  These operations are carried out by means of a number of hydraulic rams, which regularly decrease in size.  Fig. 3 roughly represents one of these rams with the plunger ready to descend and force its way into the partially formed red hot gas cylinder, C, and further into the well, W. The plunger may be compared to a finger and the cylinder to a glove, while the well may represent a hole into which both are thrust in order to reduce the thickness of the glove.  With huge tongs the cylinder, fresh from the furnace, is placed in position, but just before the plunger presses into the red hot cup, one of the workmen empties into the latter a little water, so as to partially cool the bottom and prevent its being thrust out by the powerful plunger.  Oil is also used plentifully, so that as the plunger works slowly down the red hot mass, it is surrounded by smoky flames.  It presently forces the cylinder into the well, and when the end of the stroke is reached, a stop piece is inserted through an opening in the upper part of the well, so as to arrest the edge of the cylinder while the reverse action of drawing out the plunger is proceeded with.  Directly the finger is drawn out of the glove—­in other words, immediately the plunger is raised out of the cylinder—­the latter drops down below with a heavy thud, still in a red hot condition.

[Illustration:  FIG 3.]

This operation of hot drawing is repeated again and again in rams of diminishing size until the cylinder assumes the diameter and length required.  This hot drawing leaves the surface of the metal marked with longitudinal lines, not unlike the glacier scratches on a rock, albeit they are straighter and more regular.  But the next operation not only obliterates these markings, and gives the metal a smooth surface like that of polished silver, but it also confers upon the material a homogeneity which it did not before possess, and without which it would never bear the pressure which it is destined to withstand when finished.  This operation consists in a final application of the hydraulic ram while the metal remains perfectly cold, instead of red hot, as in the previous cases.