moved provided on both of the long sides 104 and 105 of the platform 101 and extending the full length of the platform are recesses 106 in each of which a rail 102 for the sidewalls of the hoods 45 and 46 is permanently attached to the platform 101 under the platform 101 and at predetermined uniform spacing over the whole length are bearings 108 which support arms 109 which are permanently attached to the second moveable rail 103 the second rail 103 is mounted parallel to the first rail 102 and likewise extends the whole length of the platform 101 the supports 109 for the rails 103 are mounted in the bearings 108 in a horizontal plane perpendicular to the long axis of the platform 101 such that they can slide back and forward in response to a facility 110 to which they are connected the provision of the stationary and moveable rails 102 and 103 on each side of the platform 101 allows like the arrangement and shape of the rails 55 and 56 in the second exemplified embodiment of the invention the hoods 45 and 46 to be moved as described below as the sidewalls of one hood are supported on rollers.