is not illustrated the underside of cover 10 is covered with a strip of nylon which engages with guide surfaces of guide rails 13 a and 14 a guide rails 13 a 13 b 14 a and 14 b run parallel to each other and either side of opening 3 guide rails 13 a and 14 a are adjacent to the inside face of cover 10 and guide rails 13 b and 14 b are adjacent the outside face of cover 10 cover 10 also has lateral projection 12 which is folded down over guide rail 13 a to engage it and assist to guide cover 10 apparatus 1 is connected to the silo not shown by means of bolts which pass through holes not shown such as through stabilising member 16 guide rails 13 a and 14 a as well as a further stabilising member mounted approximately across the centre of the apparatus not shown bolts are placed through holes 6 and 8 and similar holes on guide rail 14 a to attach the upper guide rail 13 b or 14 b to the lower guide rail 13 a or 14 a fig3 and 4 depict plan views of the apparatus of fig1 and 2 lateral projection 12 is visible as it projects slightly beyond the lateral surface of guide rail 13 a in order to engage it although it is not shown a similar lateral projection engages guide rail 14 a fig5 and 6 depict side views of the apparatus as shown in fig1.