There was a formula by which circumference could be determined from an arc, she was quite sure of it. She had forgotten what the damned thing was, that was the only problem. But she could maybe get a rough idea – always assuming her impression of just how much the thing's edge curved was accurate – by estimating the thing's center point …
Bobbi went back to the hall table and opened its middle drawer, which was a sort of catch-all. She rooted past untidy bundles of canceled checks, dead C, D, and 9-volt batteries (for some reason she had never been able to shitcan old batteries what you did with old batteries was throw them in a drawer, God knew why, it was just the Battery Graveyard instead of the one the elephants were supposed to have), bunches of rubber bands and wide red canning-rubbers, unanswered fan letters (she could no more throw out an unanswered fan letter than a dead battery), and recipes jotted on file-cards. At the very bottom of the drawer was a litter of small tools, and among them she found what she was looking for – a compass with a yellow stub of pencil sleeved into the armature.
Sitting in the rocker again, Anderson turned to a fresh sheet and drew the leading edge of the thing in the earth for the third time. She tried to keep it in scale, but drew it a little bigger this time, not bothering with the surrounding trees and only suggesting the trench for the sake of perspective.
'Okay, guesswork,' she said, and dug the point of the compass into the yellow legal pad below the curved edge. She adjusted the compass's arc so it traced that edge fairly accurately – and then she swept the compass around in a complete circle. She looked at it, then wiped her mouth with the heel of her hand. Her lips suddenly felt too loose and too wet.