The form of the proglottides is most generally a rhombic or trapezoidal figure.
Near the middle of the long side is an opening; and from it a flight of seven steps led down to a trapezoidal chamber, on the back wall of which are two lions' heads of bronze, through which water, conducted in long semi-cylindrical channels of bronze, from behind the wall, poured out into pitchers for which holes are cut in the floor.
This is called the trapezoidal or chordal area, and will be denoted by C1.
The alternative method, therefore, consists in taking a simple formula, such as the trapezoidal rule, and correcting it to suit the mutual relations of the differences.
As the measurements can only be rough, the trapezoidal rule is the most appropriate in ordinary cases.