device mounted thereon is set in place in the normal manner care being taken to see that the cover is centred on the well so that the abutment surface 4 will abut against the wall of the well as the cover is mounted in place the arm or lever 5 will with its outer end slide towards the well wall and when the cover is firmly in place assume an obliquely upwardly sloping position the cover is thereby locked if the cover is lifted the outer end 6 of the arm 5 will be urged against the well wall the arm 5 actuating by the intermediary of the journal 3 the sleeve 1 and urging its abutment surface 4 against the opposite wall the size of the abutment forces increases by the force with which the cover is lifted the locking device is inactuated in that a rod 8 adapted to fit in the opening of the sleeve 1 and serving as an operating member is pushed down into the sleeve so that it strikes the end 7 of the arm 5 and pushes it downwards the engagement between the arm end 6 and the well wall then ceases and the cover can be lifted in certain types of covers the fitting of the cover so that a fixed abutment surface comes into abutment with the well wall can be troublesome in such wells or covers the embodiment according to fig2 can be used.