Because the early methods of priming left the vent open when the cannon fired, the little hole tended to enlarge. Many cannon during the 1800's were made with two vents, side by side. When the first one wore out, it was plugged, and the second vent opened. Then, to stop this "erosion," the obturating (sealing) primer came into use. It was like the common friction primer, but screwed into and sealed the vent. Early electric primers, by the way, were no great departure from the friction primer; the wires fired a bit of guncotton, which in turn ignited the powder in the primer tube. MODERN USE OF BLACK POWDER Aside from gradual improvement in the formula, no great change in powder making came until 1860, when Gen. Thomas J. Rodman of the U. S. Ordnance Department began to tailor the powder to the caliber of the gun. The action of ordinary cannon powder was too sudden. The whole charge was consumed before the projectile had fairly started on its way, and the strain on the gun was terrific. Rodman compressed powder into disks that fitted the bore of the gun. The disks were an inch or two thick, and pierced with holes. With this arrangement, a minimum of powder surface was exposed at the beginning of combustion, but as the fire ate the holes larger (compare fig. 20f), the burning area actually increased, producing a greater volume of gas as the projectile moved forward. Rodman thus laid the foundation for the "progressive burning" pellets of modern powders (fig. 20). [Illustration: Figure 20--MODERN GANNON POWDER. A powder grain has the characteristics of an explosive only when it is confined. Modern _propellants_ are low explosives (that is, relatively slow burning), but _projectiles_ may be loaded with high explosive, a--Flake, b--Strip, c--Pellet, d--Single perforation, e--Standard, 7-perforation, f--Burning grain of 7-perforation type. Ideally, the powder grain should burn progressively, with continuously increasing surface, the grain being completely consumed by the time the projectile leaves the bore, g--Walsh grain.] For a number of reasons General Rodman did not take his "perforated cake cartridge" beyond the experimental stage, and his "Mammoth" powder, such a familiar item in the powder magazines of the latter 1800's, was a compromise. As a block of wood burns steadier and longer than a quick-blazing pile of twigs, so the 3/4-inch grains of mammoth powder gave a "softer" explosion, but one with more "push" and more uniform pressure along the bore of the gun. It was in the second year of the Civil War that Alfred Nobel started the manufacture of nitroglycerin explosives in Europe. Smokeless powders came into use, the explosive properties of picric acid were discovered, and melanite, ballistite, and cordite appeared in the last quarter of the century, so that by 1890 nitrocellulose and nitroglycerin-base powders had generally replaced black powder as a propellant.