in a substantially vertical direction and lower cam slot 18 is slightly inclined in a substantially horizontal direction in this manner the invention provides for the two required positions of the bottom member in order to avoid inadvertently pushing objects outside the reach of the apparatus passage of the bottom member from the vertical retracted position to the extended horizontal position is effected when collection of objects is completed as may be seen in fig1 and 2 a protective grid 20 slanting frontwardly from the apparatus is mounted over the whole length of the wall the grid operates as an anti spilling device when objects are captured between the side walls in their closed position the side walls can assume various shapes as in the example under discussion they can be folded in their middle portion so as to include an angle of about 135 u00b0 with their ends substantially perpendicular to rear wall 1 in their opened position and substantially parallel to rear wall 1 in their closed position so as to entrap and precompress the collected objects however the walls can also be of a general curved shape following any required curvature rear wall 1 may also be of a curved shape so as to present in its lower portion an efficient angle of attack the side wall hinges may be mounted on the rear wall which may further have flanges connected to its front face at right angles the flanges may be formed in their lower portion with the cam slots 17 and 18 and may.