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." [Illustration: A SECTION OF THE MIDVALE STEEL CO. PLANT, SHOWING TYPES OF 6-INCH, 7-INCH, AND 8-INCH, NICKEL-STEEL, BREECH-LOADING RIFLES.] [Illustration: THREE FINISHED 8-INCH RIFLES, 45 CALIBERS, SET UP ON TURRET MOUNTS IN THE PLANT OF THE MIDVALE STEEL CO. WHERE THEY WERE MADE. In the foreground are three large gun tubes partially completed. This company started the manufacture of the first piece of ordnance material in America in 1880.]