Cover device and blanket for covering liquid surfaces

A cover device for covering a surface of a liquid in a pond, having a shell (2) defining an inner cavity, and the inner cavity is to be filled at least partly with liquid, wherein the inner cavity is divided into at least two chambers (3, 4) by a separating means (5, 51, 52), and a first chamber (4) of the at least two chambers is provided by a first opening (6) formed in the shell (2), and a weight member (8) is arranged in proximity of said first opening (6), said weight member (8) is provided with a through hole (8a), and arranged in the first opening (6) of said shell (2), wherein the device has a form substantially of a sphere, and the area of the separating means (51) as measured perpendicularly to an axis (A) of the device (1), passing through the first opening (6) and the separating means (5, 51, 52), is smaller than a greatest internal cross sectional area of any chamber (3, 4) as measured perpendicularly to the axis (A).