Equilibrium of a Knife in Mid-Air
Be reassured dear readers, we are not going to ask you to make a balance in mid air, that would be too much for our weak capabilities. The question is simply to swing a knife horizontally in the space which surrounds us. The experiment is curious and easily executed.
Take the cork of a champagne bottle. Pierce it lengthwise with a sharp knife, and let the knife stick out a third of its length from the thin end of the cork. Then insert into each side of the cork the prongs of two forks, so that they are perpendicular with the blade of the knife as shown in figure.
This operation accomplished, you have only to suspend the point of the blade on the loop of a string, and the knife will hang horizontally. You may then swing it if you choose, and the movement will not destroy the equilibrium.