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 ex