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, and at the smallest diameter of the chase, 7/16 or 2-5/8 inches. This table compares the three degrees of fortification used in Spanish culverins: Wall thickness in 8ths of caliber Vent Trunnion Chase Bastard culverin 7 5 3 Legitimate culverin 8 5-1/2 3-1/2 Double-fortified culverin 9 6-1/2 4 As with culverins, so with cannon. This is Collado's table showing the fortification for Spanish cannon: Wall thickness in 8ths of caliber Vent Trunnion Chase Canon sencillo (light cannon) 6 4-1/2 2-1/2 Canon comun (common cannon) 7 5 3-1/2 Canon reforzado (reinforced cannon) 8 5-1/2 3-1/2 Since cast iron was weaker than bronze, the walls of cast-iron pieces were even thicker than the culverins. Spanish iron guns were founded with 300 pounds of metal for each pound of the ball, and in lengths from 18 to 20 calibers. English, Irish, and Swedish iron guns of the period, Collado noted, had slightly more metal in them than even the Spaniards recommended. [Illustration: Figure 25--SIXTEENTH CENTURY CHAMBERED CANNON. a--"Bell-chambered" demicannon, b--Chambered demicannon.]