The objection to the present method of forging shells is as a result, the grain or fibre of the metal lies parallel with the major axis of the forging, the forging process causing an elongation of the ingot and the metal grain following the direction of elongation. Consequently any flaws occurring in the material will extend parallel to the grain or major axis. If a flaw remains undiscovered in a finished projectile--as is sometimes the case--the projectile is not only weakened thereby, but the element of weakness lies in such a direction that the compression forces and counterforces produce very much the same results as would a wedge driven into a niche, i.e. the separation of adjacent material. The author is in possession of a shell in which a longitudinal flaw was revealed in the ogive by the cutting away of a longitudinal quarter section, Fig. 28. [Illustration: Fig. 28. Armor-Piercing Shell. Showing position of flaw.] There are, therefore, two great forces with which to contend in the design of projectiles, to one of which, compression, has been given the greatest attention because of its recognized tendency to cause the base of the shell to crowd upon the head and cause the shell to break up about the ogive. The other force, torsion, seems not to have been considered prior to the present instance, at any rate so far as the author has been able to ascertain, not because thought to be unimportant, but because of oversight or failure on the part of investigators to take into consideration in this instance, an element of reaction commonly considered in mechanical engineering practice, as in shafting for vessels and for power transmission in shops, etc. The writer maintains that immediately upon impact the metal in a shell assumes a state of physical unrest, due to stresses similar to those in a propeller shaft when in motion, except that in the former case the intensity of the compression stresses greatly exceed those in the latter. Because a shell is only 3-1/2 calibres in length is no criterion that the same stresses do not exist there as would exist in the theoretical projectile considered of twenty calibres, or one of even more exaggerated proportions--there would be merely a difference in the _intensity_ of these stresses. In a projectile making one complete revolution about its major axis in every twenty-five calibres flight, any one elementary unit area or mass in that shell likewise makes one complete revolution in the same distance of travel, and the path traversed by that unit area or mass is that of a spiral of radius equal to the distance of that unit area or mass from the major axis of the shell, the diameter of which spiral would be the diameter of the shell in question--and the pitch twenty-five calibres--if said unit area were on the surface of the body of the shell.