Greenhouses, their construction and equipment . IRRIGATION 241 satisfactorily on a* water pressure of fromlo to 30 pounds. When constructing benches for sub-irriga-. Fig. 126.—Greenhouse bench arranged for , cement troughs on bottom of bench; B, drain tile orperforated pipes for supplying water; C, drainage spacesbetween troughs. tion, the essentials are a water-tight bottom,usually of cement, to prevent the water fromleaking through, and perforated pipes or tilesfor distributing it along the bench. Thissystem has been tried out extensively withvarying results by the Ohio exper
Greenhouses, their construction and equipment . IRRIGATION 241 satisfactorily on a* water pressure of fromlo to 30 pounds. When constructing benches for sub-irriga-. Fig. 126.—Greenhouse bench arranged for , cement troughs on bottom of bench; B, drain tile orperforated pipes for supplying water; C, drainage spacesbetween troughs. tion, the essentials are a water-tight bottom,usually of cement, to prevent the water fromleaking through, and perforated pipes or tilesfor distributing it along the bench. Thissystem has been tried out extensively withvarying results by the Ohio experimentstation. CHAPTER XV CONCRETE CONSTRUCTION Concrete is a combination of Portlandcement, sand, crushed stone or gravel andwater, thoroughly mixed and then allowedto set or harden. Portland cement, or cement, as it is nowcommonly known, is manufactured by burn-ing and grinding together- limestone andclay, or shale, in certain proportions. It de-rives its name, Portland cement, from its re-semblance to Portland stone. It is alsosometimes known as hydraulic cement, orbuilding cement. Concrete has wellnigh revolutionizedbuilding practice in the last 25 years
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