A survey is not a plan. It's a list of numbers you haven't yet forced to agree with one another, and it takes two or three nights at the table to find out which of them lie. I started Tuesday evening, at home, calçada do Monte, the window open onto the roof tiles and the aerials and the sound of the 28 braking down below. The first eleven niches fell into place on their own. Thirty degrees each, give or take two or three minutes of arc, which is very fine work for a seventeenth-century chisel. Twelve times thirty makes three hundred and sixty. A dodecagon is a docile figure.
The twelfth refused to close. Thirty-four degrees, and not thirty-four through any fault of mine: I had taken three sightings from three different stations, and all three gave the same discrepancy. Then came the rest. The eleven niches are sixty-two centimeters deep, to within a centimeter, all the way around the room. The twelfth is forty-seven. Its back isn't on the circle: it bulges toward north, the bulge of a hole cut into a wall that was already finished, by someone with no room to step back.
I went back to my photographs and looked at the tool marks. The eleven were cut with a broad bevel, in even passes, right-handed, with a quarryman's tool that knew what it was doing. The twelfth was attacked with the point, in small hurried blows, and in two places the edge blew out and nobody came back to make it good. You don't cut a twelfth place in a room built for twelve. You cut it in a room built for eleven, on the day you suddenly have to put one more person away.