[In the case of] a beam that was placed from one wall to another [and] there is tu-mah underneath it: if there is a handbreadth, [it] spreads tumah underneath its en-tirety. If not, tumah penetrates upward and downward. How large a perimeter will have a width of a handbreadth? If [the beam] is round, its perimeter must be three handbreadths. If it is square, [its perimeter must be] four [handbreadths]; because a square exceeds a circle by one quarter.