A description briefly my invention comprises a cap for a grain bin the cap being operable by linkage means from ground level more specifically and referring to the drawings the device is intended to be mounted on a grain bin 10 having a conical roof 11 at the apex of the roof an opening is defined by a rim 12 it is through this opening that the ventilating air current will flow a cap 13 is attached to either the roof 11 or the rim 12 by a hinge 14 so that it can be pivotally moved from an open position fig1 to a closed position fig2 and back again in order to control the movement i provide a linkage and cable mechanism as shown in fig1 and 2 the linkage includes an arm 15 fixed to the cap 13 and having a pin 16 providing a pivot point substantially spaced in a vertical direction from the cap 13 when the cap is closed a similar operating arm 18 is pivoted to a bracket 19 mounted on the roof 11 a joining link 20 pivotally attached to the arms 15 and 18 by the pin 16 and a second pivot pin 21 respectively completes a linkage which will provide positive movement to move the cap 13 either to open.