| 摘要 |
<p>A flared peripheral wall has an inner end internal flange forming the margin of a vent opening and an outer end external flange. Ribs projecting outward from the peripheral wall can be embedded in concrete to prevent displacement of the wall. Screen cloth covering the vent aperture is secured to the inner end internal flange. The vent aperture is closable by a sheet cover having resilient retaining flanges or by a plug of foam plastic.</p> |
