"One of the burglars sat on the couch there, rubbed plaster on the sole of his boot, and set his foot down on the carpet.

Then he dusted the rest of the plaster off his boot and put the book on the top of the footprint." "Now, how do you know that ?" said the astonished Duke.
"It's as plain as a pike-staff," said Guerchard.

"There must have been several burglars to move such pieces of furniture.

If the soles of all of them had been covered with plaster, all the sweeping in the world would not have cleared the carpet of the tiny fragments of it.

I've been over the carpet between the footprint and the window with a magnifying glass.