ACCELERATING VEGETATION. 



393 



be used without any preparation, by which much heat is gained ; and that 

 in the winter months, when a powerful artificial heat is required, which (in 

 the case of common hot-beds) is apt to burn the plants, they are here in 

 the coldest part of the soil, and cannot possibly be injured by any degree of 

 heat, which can be communicated by dung. Fig. 327 is a section of a pit on 



Fig. 327. Cross section of a pit on M'Phail's principle, with variations. 



this principle, with some improvements : a a is the surface of the ground ; 

 b 6, excavations for the dung-casings, 2-L feet deep, 18 inches wide at bottom, 

 and 2 feet wide at the ground's surface ; the greater width at top being to 

 prevent the dung from shrinking from the side of the excavation as it sinks ; 

 c is the outer perforated wall, a brick in width ; d, the inner wall of brick 

 set on edge, and tied to the outer wall with occasional cross bricks ; e, is a 

 layer of billet wood 1 foot in thickness to admit of the heat penetrating from 

 each side, or the same object may be effected by a layer of loose stones ; f, a 

 covering of fagot wood, over which a layer of turf or litter is placed to pre- 

 vent the soil from sinking into and choking up the interstices in the layer of 

 billet wood ; g, the bed of soil ; /*, a trellis for vines, melons, or other plants, 

 at one foot from the glass ; i, a gutter for receiving the water from the glass, 

 and which should conduct it through a small pipe, either at one end, or in 

 the middle to a small barrel, or to a cistern of slate or other material sunk 

 in the soil of the pit in front. The preferable situation is mid way between 

 either end, in order that the vapour of the water may be equally diffused in 

 the atmosphere of the pit. By keeping the upper surface of the dung of the 

 form shown in the figure, it will throw off the rain, which may be conducted 

 away in small surface gutters. 



844. The formation of common hotbeds is effected by first making out the 

 dimensions of the bed, which should be six inches wider on all sides than 

 that of the frame to be placed over it, and then, by successive layers of dung 

 laid on by the fork, raising it to the desired height, pressing it gently and 

 equally throughout. In general, such beds are built on a level surface ; but 

 Mr. Knight's forms a surface of earth as a basis, which shall incline to the 

 horizon to the extent of 15; on this he forms the dung-bed to the same in- 

 clination ; and, finally, the frame, when placed on such a bed, if, as is usual, 

 it be deepest behind, will present its glass at an angle of 20, instead of six or 

 eight, which is undoubtedly of great* advantage in the winter season. This 

 seems a very desirable improvement where light is an object, which it must 



