I found a drawing of a military engineered fourgasse. Without the drawing the description of Anderson's fourgasse made no since. It was 12 feet in diameter with 50 pounds of powder. Look at the drawing at the link below. The fourgasse was an angled hole, twelve feet in width, filled with something, either pieces of stone and iron or artillery shells, with a contained charge at the bottom to propel the contents out, like a cluster bomb. Very interesting.

http://books.google.com/books?id=Uz1HAAAAIAAJ&pg=PA91&dq=Fougasse+explosive#PPA91,M1

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David Upton