Grant wrote:
"Cannon used to be named by the weight of the iron roundshot they
fired, and these went up in 4-pound intervals. In sailing ships of
the early nineteenth century, 24-pounders constituted much of the
broadside, so it was a pretty common calibre of gun. This method of
naming was used across Western Europe and the US - and since a
Spanish pound ("libra") was 1.01 English pounds, the size of gun must
have been pretty much identical to the British version.
Certainly in English usage it was acceptable to call such a gun "a
24" or "a long 24" (for the long-barrelled version), so I'd guess
you're seeing the Spanish equivalent here.
If memory serves these guns could throw a round shot a mile (1.6km) -
can't remember if this was including or excluding bouncing of the
shot. Would this accord with the width of the boundary defined by the
treaty?"

I don't know - I don't have a good enough map for that to judge. But
it does seem that the range of the cannon was only a starting point
for the engineers to establish the boundary. Cf. this map from 1891,
which neatly shows the neutral zone, too. The map can be enlarged (a
bit) by clicking on the magnifying glass icon.

Peter S.