The membrane cover has fold lines therein intersecting the walls of the
reservoir at intersection points. A geometric chord or secant joins two
intersection points, and defines, with a portion of the walls of the
reservoir, a static segment in the membrane cover. The membrane cover in
the static segment is tensioned along lines of force that are parallel to
or that make acute angles, with the aforesaid chord or secant. The
membrane cover in the static segment remains substantially planar and
stationary, thus static, despite the rising and lowering of the remaining
portion of the cover in response to variations in the level of the
content of the reservoir. A hatchway is mounted in the static segment of
the cover. In another aspect of the invention, the hatchway has an
airtight compartment extending between the membrane cover and a point
below the level of the material inside the reservoir.