put into a culverin, for the stone projectile
weighed only a third as much as an iron ball of the same size, and the
bore walls could therefore be comparatively thin. They were made in
calibers up to 50-pounders. There was a chamber for the powder charge
and little danger of the gun's bursting, unless a foolhardy fellow
loaded it with an iron ball. The wall thicknesses of this gun are
shown in Figure 24, where the inner circle represents the diameter of
the chamber, the next arc the bore caliber, and the outer lines the
respective diameters at chase, trunnions, and vent.

[Illustration: Figure 24--HOW MUCH METAL WAS IN EARLY GUNS? The charts
compare the wall diameters of sixteenth-seventeenth century types. The
center circle represents the bore, while the three outer arcs show the
relative thickness of the bore wall at (1) the smallest diameter of
the chase, (2) at the trunnions, and (3) at the vent. The small arc
inside the bore indicates the powder chamber found in the pedrero and
mortar.]

Mortars (fig. 23d) were excellent for "putting great fear and terror
in the souls of the besieged." Every night the mortars would play upon
the town: "it keeps them in constant turmoil, due to the thought that
some ball will fall upon their house." Mortars were designed like
pedreros, except much shorter. The convenient way to charge them was
with _saquillos_ (small bags) of powder. "They require," said Collado,
"a larger mouthful than any other pieces."

Just as children range from slight to stocky in the same family, there
are light, medium, or heavy guns--all bearing the same family name.
The difference lies in how the piece was "fortified"; that is, how
thick the founder cast the bore walls. The English language has
inelegantly descriptive terms for the three degrees of
"fortification": (1) bastard, (2) legitimate, and (3)
double-fortified. The thicker-walled guns used more powder. Spanish
double-fortified culverins were charged with the full weight of the
ball in powder; four-fifths that amount went into the legitimate, and
only two-thirds for the bastard culverin. In a short culverin (say, 24
calibers long instead of 30), the gunner used 24/30 of a standard
charge.

The yardstick for fortifying a gun was its caliber. In a legitimate
culverin of 6-inch caliber, for instance, the bore wall at the vent
might be one caliber (16/16 of the bore diameter) or 6 inches thick;
at the trunnions it would be 10/16 or 4-1/8 inches, an