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Coehorn Mortar
from "The Mortar" by WL Ruffell

Baron van Coehorn (1641-1704), a Dutch Officer, designed a mortar first used in 1674 which fired at a fixed angle of elevation of 45°. Range was adjusted by varying the weight of the propellant charge, or by moving the mortar (see Fig. 2 on previous page) - providing it was one of the lighter equipments! See Fig. 4. Britain adopted the Coehorn mortar shortly after its invention.

Bronze Coehorn mortar
Calibre: 4.5 inches (114 mm)
Weight (piece): 86 lbs
Length (piece): 13 inches.
Made in 1814 and still on its original bed this one is very similar to the one in the Army Memorial Museum, Waiouru.
Coehorn mortar
Figure 4

This Coehorn is designated 'mortar 4 2/5th inch' because mortars were designated by the diameter of the shell fired. Mortars did not fire shot. The Coehorn mortar illustrated is similar to those used by the New Zealand Armed Constabulary during the New Zealand Wars of the 1860s.

To lay for line the layer stood behind the mortar with a plumb line in his hand and directed movement of the bed or mounting until the plumb line and the line of the center of the mortar were in line with the target. This method was still in use as recently as World War 1.

Laying 19th century 13-inch (330-mm) mortar for line Laying 19th century mortar
Figure 5

Mortars fired shell only. At first the firer ignited the fuze with a port-fire or linstock thrust down the barrel, then quickly fired the mortar itself, at the same time prayingn to St Barbara that the latter would not misfire! Tradition has it that in 1689 at the Siege of Limerick a Lieutenant Fireworker Brown 'in consequence of an accident' fired a mortar without lighting the fuze first - but found it functioned anyway, ignited by the flash from the propellant charge. Thus a Gunner hazard disappeared.

By 1750 mortars had become standardised and limited to the following: 4 2/5th inch (Coehorn), 5.5-inch (Royal), 8-, 10-, and 13-inch in bronze and 8-, 10-, and 13-inch in cast iron. The Navy made use of the heavier 10- and 13-inch types in 'bomb' vessels. Except for size all bore a close resemblance to the Coehorn shown above and were still current equipment in some British colonies as recently as 1924!

For use in the field the heavier pieces were mounted upon carriages.

SBML 10-inch on travelling carriage limbered up
Figure 6a
SBML 10-inch on travelling carriage limbered up

Shown below is the sequence of coming into action for a 10-inch mortar. Prior to occupying a position a level platform was prepared.

Having been unlimbered on the prepared platform the mortar was 'turned on its nose' and the wheels removed.

Turned on its nose and wheels removed
Figure 6B

The wheels having been removed, the carriage was lowered to the ground.

Lowered to the ground
Figure 6C

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