Abstract: A pool cover sheet has beaded edges guided in side edge guides along the pool. The cover sheet is wound onto and pulled from a spool mounted on a motor driven shaft. The cover sheet is pulled by the spool by cords traveling along the side edge guides with cords being wound onto and from cord drums.The velocity of the cover sheet travel is attempted to be matched to the cover sheet velocity by means frustoconical drums having tapered surfaces onto which the cords are wound and unwound. To attempt to equalize the tension in the cords as well as in the cover sheet, the spools and drums are driven through opposed torsion springs from a common main shaft. The tension in the respective sides of the sheet or in the respective cords may be adjusted to take up any slack found to occur.

Abstract: A greenhouse structure is formed with a substantially flat and horizontal roof formed of a plurality of roof sections each pivoted about a pivot or hinge end to swing upwardly from a closed position to an open position to admit ambient air to flow into or from the interior of the structure to cool the same. The roof sections are lifted by a lift means which becomes part of the roof supporting structure, and, to this end, the lift means is associated with the vertical support columns. Preferrably, the lift means comprises elongated straight lift members each carried by and stabilized by an associated support column for traveling along vertical and rectilinear paths of travel. The preferred means for shifting the lift members and thereby the roof sections comprises a rotatable nut in threaded engagement with the lift member to shift the same vertically as the nut is turned by a worm gear means.