A Sure Eye
Stick a sharply-pointed knife, preferably with a heavy handle, into the lintel of the door, or into a wooden beam upon the ceiling. Then ask for some one with a sure eye to mark the exact spot on the floor by putting a cent upon it, where the knife will drop when displaced.
This is easily done in the following manner. Take care that the knife is very lightly stuck into the wood, so that it will fall at the slightest touch.
Fill a glass with water, and raising it to the knife immerse the handle to the height of an inch or so. Then watch where the drop of water, which will fall when you remove the glass, strikes the ground, place the cent upon the spot, and you will find that the knife will fall upon that identical place.