The number of cylinders employed generally increases, up to a certain limit, with the size of the gun, practical considerations governing; and the "shrinkage," or amount by which the inner diameter of the outer cylinder is less than the outer diameter of the one which it is to be shrunk over, is a matter of nice calculation. Roughly speaking, it is about one and one half one-thousandths of an inch for each inch of diameter, varying with the position of the cylinder in the gun; and its accurate attainment, throughout the length of the cylinder of a large gun, is a delicate matter of the gun-maker's art and the machinist's skill.
The method of assembly is to have the cold tube set upright and prepared for a circulation of water within the bore of the tube to keep it cool. Then the hoop, whose inside diameter is smaller than the outside diameter of the tube on which it is to be shrunk, is measured and carefully heated to a temperature of approximately 450° F., or just about the temperature of a good oven for baking or roasting. This mild temperature so expands the material in the hoop that the difference of diameter is overcome and the hot hoop is expanded to a larger inside diameter than the outside diameter of the cold tube on which the hoop is to be placed. Next the hot, expanded hoop is placed in position around the breech end of the tube, and slowly and carefully cooled, so that in contracting from the high temperature to the low ordinary temperature, the hoop shrinks toward its original diameter and thus exerts an inclosing pressure or compressive strain upon the breech end of the tube.
Now when the gun is fired the tube tends to expand under the pressure and this expansion is resisted, first by the compressive force exerted by the shrunken hoop and later by the hoop itself, so that the built-up system is stronger and better able to resist the explosive charge of the burning powder than would be the case if the gun were made in one piece and of the same thickness of metal.
This brief explanation will show why so many pieces are provided for the manufacture of the finished gun and the reason for the large number of machine tools and machining operations necessary in order to carry forward the manufacture of the finished article. Sometimes one or more of the outer cylinders are replaced by layers of wire, wound under tension.
Both our 4.7-inch gun, model 1906, with which our troops have been equipped for a long time and which throws a projectile weighing 45 pounds a distance of about 6 miles, and the French 75-millimeter (2.95-inch) gun, successfully used by the French since 1897, were designed to be drawn by horses, and the guns are best used when drawn by teams of 6 or 8 horses. As the horse has a sustained pulling power of only 650 pounds, it is obvious that the weight to be drawn by the team of 6 horses must not be more than 3,900 pounds. So there is every incentive for making mobile artillery of this kind as light as possible, consistent with the strength required for the work to be done. Thus the pulling power of the horse coupled with his speed has been the limiting factor in the design and weight of mobile field artillery.
As one of our foremost United States ordnance engineers once said, "the limited power of the horse is what has governed the weight of our artillery," and that "if Divine Providence had given the horse the speed of the deer and the power of the elephant, we might have had a far wider and more effective range for our mobile artillery."
A SECTION OF THE MIDVALE STEEL CO. PLANT, SHOWING TYPES OF 6-INCH, 7-INCH, AND 8-INCH, NICKEL-STEEL, BREECH-LOADING RIFLES.
THREE FINISHED 8-INCH RIFLES, 45 CALIBERS, SET UP ON TURRET MOUNTS IN THE PLANT OF THE MIDVALE STEEL CO. WHERE THEY WERE MADE.