GREEN-HOUSE STRUCTURES. 59 



front wall 9 feet apart. The proper position of these 

 sliding shutters is of importance. It has long been a 

 common practice to ventilate by front sashes, both in 

 graperies and plant houses which opened directly on the 

 plants. The consequence is, that in cold days, while the 

 sun is shining, the house gets heated up, but the lower 

 sashes cannot be opened, for the outside air is often so 

 cold that it would be likely to chill or freeze the plants. 

 But by placing the sliding ventilators on the front or 

 south wall, at such a height that they will be below the 

 front bench, but above the flue or pipes, the air thus ad- 

 mitted to cool down the house is modified in its passage 

 across the pipes or flue, so that the temperature of the 

 house is lowered without chilling the plants. 



The sliding ventilators run upon rollers, and by at- 

 taching a continuous rod to each, 25 or 30 of them can be 

 moved from one end by a slight 

 pull, thus obviating the necessity 

 of getting under the benches to 

 open each one separately. The 

 front or south wall of the building 

 referred to is built of brick-work 8 

 inches thick. But the back or 

 north wall is built hollow, 10 Fig. 18. HOLLOW BRICK 

 inches thick; that is, of two WALL. 



bricks placed flat, with an air space of 2 inches, " tied in " 

 by headers of cast-iron, like that shown in figure 18, placed 

 at intervals of 2 feet apart, both in the height and in the 

 length of the wall. A 10-inch hollow wall built in this 

 way costs about the same as a 12-inch solid wall, but is 

 greatly better in all respects. The air space, of course, 

 acting as a non-conductor, keeps the house much warmer 

 than a solid Avail would ; and besides, a wall like this is 

 much more durable for such a purpose. The experience 

 of most of us who have built solid 8-inch brick walls for 

 green-houses is, that on the north or west side they stand 



