| 摘要 |
<p>A method of displacing a cover sheet over an open area to be covered. The method comprises providing a plurality of displaceable cylinders in a pair of channels extending parallel to one another on a respective side of the open area. The cylinders are secured to the cover sheet to displace it between the pair of channels. Water under pressure is applied to the cylinders to displace them in a predetermined manner relative to one another to cause the sheet to be displace over and away from the open area.</p> |
