Putting shells on target: Aiming the siege mortars

Earlier posts about mortars have focused on the weapons, projectiles, mountings, and means of transportation.  Let me turn now to some discussion about how the weapon was used.  Mortars typically sat in “pits” which precluded direct observation of the target.   Sighting the mortars was not like the practice used for direct fire weapons (such as guns and in most cases howitzers… but hold onto that last one for a few paragraphs here…). For the direct fire weapons, the gunner’s primary input to the “fire solution” was the line of sight from breech to muzzle, using the sights to point the weapon onto the target.  That could not be done in most cases where mortars were employed.

Mortars used indirect firing methods.  And recall that unlike what the “coffee table” histories tell us, indirect fire was a common practice by the time of the Civil War … centuries old in fact. For seacoast mortars, the garrison had plenty of time to lay out firing reference data.  The distance between the mortar’s position and the ship channels were well known.  The crew had the opportunity to calculate the firing solutions and re-calculate where needed.  All was then recorded for use when called for.

But for siege operations, regardless of what type of mortar was used, the crew lacked the opportunity to measure and calculate firing solutions to such a fine degree of accuracy.  Instead, the crew needed to factor that firing solution in the field as part of the mortar’s setup.   Again I turn to Colonel John C. Tidball’s Manual of Heavy Artillery Service, to describe how this was done.  Although written two decades after the Civil War, Tidball captured the practice with clarity.  And the practice had not changed much in two decades.

The practice of “aiming” a mortar relied upon some basic geometry.  To draw the most accurate line, you need more than two points of reference.  So the practical application of geometry started with the known points – the target and the mortar location.  To obtain an accurate measure of the angle on that line, which Tidball called the plane of sight, the crew used additional reference points.  Suspended below a wooden frame, a plummet became the reference point.  At a minimum, one plummet sat on the parapet of the mortar pit and another sat behind the mortar.  For precise aiming, yet another was placed just beyond the parapet in front of the mortar.  The illustration below, from the 1884 manual, provides an overview of this system of plummets.

The plummet itself hung from a wooden frame, or trestle.  Fine thread, preferably silk, suspended a weighted object (plum bob) above the ground between the trestle legs.  It was the perpendicular line formed by the string, not the weight itself, that became the reference point.  If winds worked against the force of gravity, causing the plummet to swing, the crew placed the plummet in a bucket of water to dampen movement.

The plummet behind the mortar was usually about six feet high, so the gunner didn’t need to stoop down when sighting, and about three yards behind the mortar.  That on the parapet was between a foot and eighteen inches high.  The plummet to the front had to be tall enough to be seen by the gunner from inside the pit.  Here’s Tidball’s description of the actions placing the plummets:

The gunner, assisted by No. 2, places a trestle upon the parapet near the interior crest, and suspends from it a plummet in such position that it will be approximately in the line passing through the centre of the platform and the object to be fired at.  No. 3 brings up another trestle, which the gunner causes him to place a few feet in advance of the first, and in line with it and the object; sighting by the plummet first established, he causes the second plummet [the one in front beyond the parapet] to be accurately adjusted on the line to the object; then, going to the front plummet and sighting back, he causes No. 4 to place in position the trestle in rear of the mortar, and suspended from it the plummet, being careful to have it in exact line with the two on the parapet.  The front trestle is then removed by No. 3.

Notice Tidball does not refer to the “target” but rather the “object to be fired at”.  Perhaps some Victorian sensitivities at play there.  Or perhaps a subtle choice of words which we should consider when reading the text.  After all, our “target” might not have been a target back then.

Transitioning from textbook to the field, there are several issues to keep in mind here.  Siege mortars were, by necessity, placed within range of the defending forces.  Very likely the enemy would notice the activity of placing trestles and establishing the line.  Tidball recommended the use of a priming wire, “… Should the fire from the enemy endanger the plummet on the parapet….”  Still, even that could be risky in the vicinity of sharpshooters.  Engineer Henry L. Abbot discussed other, somewhat ingenious, systems devised to work around such a tactical problem in his Siege Artillery in the Campaigns against Richmond.  I’ll save those for later posts (perhaps a 2014 sesqucentennial post?).   In his report on siege artillery, Abbot stress that mortar batteries needed good fields of view, even at the expense of additional work.

Tidball also mentioned the need for the target (or object upon which fire is placed) to be visible from the parapet.  Where that was not the case, the crew could use a system of mirrors in a more advanced application of geometry.  And by the 1880s, a device known as Paddock’s interpolator came into use.  But discussion of that device falls out of my Civil War scope.

The line of sight established, the crew could move the mortar on the platform as needed to align with the “plane of sight”.  The mortar gunner still had other factors to work out.  Not the least of which was the time of flight of the shell.  But at least the muscle work of moving the mass of iron was completed.  When you think about it, the process for establishing the azimuth of fire in the 1860s is not far removed from the practice used today by artillerists… just that now days the gunner has several computers, global positioning systems, and all sorts of other aids to rely upon.  Still, a line is a line.

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