Avoiding the most visibly French sites one gets:

Old furnace used to heat cannonballs
http://www.dabuda.net/gallery/fortSanMarco/FortSanMerco_29

The SHOT FURNACE was constructed to heat cannonballs red hot before they were fired at an enemy's wooden ships. Detail of shot furnace. ...
fcit.usf.edu/FLORIDA/photos/military/marco/marco1/marco1.htm

Directly in front of the doorway of Casemate 8 is a restored HOT SHOT FURNACE (H), which was used to heat cannonballs until they were red hot. ...
www.ils.unc.edu/parkproject/visit/foma/guide.html

The forts, with SHOT FURNACES designed to heat cannonballs and cannons capable of firing their red-hot shots up to 3 miles, could defend the coast with a ...
www.nps.gov/tourism/winter20018gp.html -

Visit Florida - Worth the Drive 2007 - Page 39
... the accoutrements of war: four bastions; a moat; a drawbridge; a SHOT FURNACE used to heat cannonballs which would, in turn, set ablaze wooden ships. ...
www.nxtbook.com/nxtbooks/milesmedia/worththedrive07/index.p...

Full details at http://www.clis.com/friends/HotShot.htm

Fort Niagra New York still has an original "HOT SHOT FURNACE" located near their "Water Battery." It's quite impressive. The cannonballs would be inserted ...
www.armchairgeneral.com/forums/showthread.php?t=44076

At Fort McHenry, an illustration of a HOT-SHOT FURNACE is provided on the wayside ... Hot Shot Furnace (Source: De Scheel's Treatise on Artillery, 1800) ...
www.bcpl.net/~etowner/bb2d.html

Fort Griswold Tour via pictures
SHOT FURNACE Built in 1843 to heat cannonballs to set ... when struck. Shot Furnace (35 KB): This is another view of the river battery's SHOT FURNACE. ...
www.revwar.com/ftgriswold/tour/



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Note added at 2 hrs (2007-08-09 16:37:56 GMT)
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It's true, Sarah, that most if not all of the references above are American, but British/Irish English terminology appears to be no different:

SHOT FURNACE Furnace for heating shot to fire at ships, to set them alight
http://www.english-heritage.org.uk/server/show/nav.001003005...

CUPOLA FURNACE
Definition A furnace used for melting metals for casting or for heating shot
http://www.imagesofengland.org.uk/help/help.asp?code=BTThes/...
(not sure if this is for heating shot for the same purpose)

The SHOT FURNACE mentioned is not evident in either the 'Proposal' drawing of 1857 (OPW 5HC/4/951), or the more reliable drawing' as executed' of 1861 (Dept. of Marine). However the War Office map of Kingstown harbour East Pierhead of 1862 (corrected 1883) marks "SHOTE FURNACE and Passage" (fig. 66).
[Dún Laoghaire Harbour Company - EAST PIER BATTERY]
http://www.dlharbour.ie/content/history/harbour/east_pier_ba...

There certainly ARE more hits on US sites. That may well be because the English military did not feel it needed shot furnaces, relying on its Navy stopping enemy ships before they got within range of shore batteries, whereas the US (and French) Navy, being inferior at the time, might have required shore batteries to back up their navies.

Rule, Britannia, Britannia rules the waves ....