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only interrupted where the tying bricks cross it, but 

 there is a free passage for air from top to bottom of 

 the wall. The front wall is four feet, and the back 

 wall five feet six inches high, enclosing a space of six 

 feet wide and fifteen feet long, and the walls are co- 

 vered with a wall-plate, and with sliding lights, as in 

 ordinary beds. The space enclosed may be filled to 

 a proper depth with leaves, or tan, when it is wished 

 to promote the rapid growth of plants. The wall is 

 externally surrounded by a hotbed composed of leaves 

 and horse-dung, by which it is kept warm ; and the 

 warm air contained in its cavity is permitted to pass 

 into the enclosed space through many small perfora- 

 tions in the bricks. At each of the lower corners is 

 a passage, which extends along the surface of the 

 ground, under the fermenting material, and commu- 

 nicates with the cavity of the wall, into which it ad- 

 mits the external air to occupy the place of that 

 which has become warm and passed into the pit. 

 The entrances into these passages are furnished with 

 grates, to prevent the ingress of vermin of every 

 kind. The hotbed is moved and renewed in small 

 successive portions, so that the temperature may be 

 permanently preserved, the ground being made to 

 ascend a little towards the wall on every side, that 

 the bed in shrinking may rather fall towards than 

 from the walls. The perforations in the interior of 

 the wall are from 18 to nearly 20 inches distant from 



