Field counterpart of the garrison cannon was the siege gun--the
"battering cannon" of the old days, mounted upon a two-wheeled siege
or "traveling" carriage that could be moved about in field terrain.
Whereas the purpose of the garrison cannon was to destroy the attacker
and his matériel, the siege cannon was intended to destroy the fort.
Calibers ranged from 3- to 42-pounders in eighteenth century English
tables, but the 18- and 24-pounders seem to have been the most widely
used for siege operations.
[Illustration: Figure 32--SPANISH EIGHTEENTH CENTURY SIEGE CARRIAGE.]
The siege carriage closely resembled the field gun carriage, but was
much more massive, as may be seen from these comparative figures drawn
from eighteenth century English specifications:
24-pounder 24-pounder
field carriage siege carriage
9 feet long Length of cheek 13 feet.
4.5 inches Thickness of cheek 5.8 inches.
50 inches Wheel diameter 58 inches.
6x8x68 inches Axletree 7x9x81 inches.
Heavy siege guns were elevated with quoins, and elevation was
restricted to 12° or less, which was about the same as United States
siege carriages permitted in 1861. It was considered ample for these
flat trajectory pieces.
Both field and siege carriages were pulled over long distances by
lifting the trail to a horse-or ox-drawn limber; a hole in the trail
transom seated on an iron bolt or pintle on the two-wheeled limber.
Some late eighteenth century field and siege carriages had a second
pair of trunnion holes a couple of feet back from the regular holes,
and the cannon was shifted to the rear holes where the weight was
better distributed for traveling. The United States siege carriage of
the 1860's had no extra trunnion holes, but a "traveling bed" was
provided where the gun was cradled in position 2 or 3 feet back of its
firing position. A well-drilled gun crew could make the shift very
rapidly, using a lifting jack, a few rollers, blocks, and chocks. When
there was danger of straining or breaking the gun carriage, however,
massive block carriages, sling carts, or wagons were used to carry the
guns.
Sling wagons were of necessity used for transport in siege operations
when the guns were to be mounted on barbette (traversing platform)
carriages (fig. 10). Emplacing the barbette carriage called for
construction of a massive, level subplatform, but it also eliminated
the old need for the gunner to chalk the location of his wheels in
order to return his gun to the proper firing position after each shot.
The Federal sieges of Forts Pulaski and Sumter were highly complicated
engineering operations that involved landing tremendously heavy
ordnance (the 300-pounder Parrott weighed 13 tons) through the surf,
moving the big guns over very difficult terrain and, in some cases,
building roads over the marshes and driving foundation piles for the
gun emplacements.
The heavy caliber Parrotts trained on Fort Sumter were in batteries
from 1,750 to 4,290 yards distant from their target. They were very
accurate, but their endurance was an uncertain factor. The notorious
"Swamp Angel," for instance, burst after 36 rounds.