When the powder bag came into use, the gunner had to prick the bag
open so the priming fire from the vent could reach the charge. The
operation was accomplished simply enough by plunging the gunner's pick
into the vent far enough to pierce the bag. Then the vent was primed
with loose powder from the gunner's flask. The vent prime, which was
not much improved until the nineteenth century, was a trick learned
from the fourteenth century Venetians. There were numerous tries for
improvement, such as the powder-filled tin tube of the 1700's, the
point of which pierced the powder bag. But for all of them, the slow
match had to be used to start the fire train.
[Illustration: Figure 18--LINSTOCKS.]
Before 1800, the slow match was in universal use for setting off the
charge. The match was usually a 3-strand cotton rope, soaked in a
solution of saltpeter and otherwise chemically treated with lead
acetate and lye to burn very slowly--about 4 or 5 inches an hour. It
was attached to a linstock (fig. 18), a forked stick long enough to
keep the cannoneer out of the way of the recoil.
Chemistry advances, like the isolation of mercury fulminate in 1800,
led to the invention of the percussion cap and other primers. On many
a battleground you may have picked up a scrap of twisted wire--the
loop of a friction primer. The device was a copper tube (fig. 19)
filled with powder. The tube went into the vent of the cannon and
buried its tip in the powder charge. Near the top of this tube was
soldered a "spur"--a short tube containing a friction composition
(antimony sulphide and potassium chlorate). Lying in the composition
was the roughened end of a wire "slider." The other end of the slider
was twisted into a loop for hooking to the gunner's lanyard. It was
like striking a match: a smart pull on the lanyard, and the rough
slider ignited the composition. Then the powder in the long tube began
to burn and fired the charge in the cannon. Needless to say, it
happened faster than we can tell it!
[Illustration: Figure 19--FRICTION PRIMER.]
The percussion primer was even more simple: a "quill tube," filled
with fine powder, fitted into the vent. A fulminate cap was glued to
the top of the tube. A pull of the lanyard caused the hammer of the
cannon to strike the cap (just like a little boy's cap pistol) and
start the train of explosions.
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.