PLANTS, WITH GLASS ROOFS. 



163 



the soil or structure to be heated, as in M'PhaiPs and other pits. A 

 steady reservoir of heat is thus provided, and instead of an extra 

 supply for unexpected cold nights, extra coverings of bast mats or 

 mats of straw are provided, for retaining heat that would escape 

 through the ordinary covering. An additional supply of heat for 

 extra cold weather may also be obtained by different means. Where 

 exterior casings of dung are employed, if the heat of the dung is 

 admitted through a pigeon-holed wall to an inside flue with thin 

 covers ; or if the dung is brought into close contact with thin plates of 

 stone or slate, instead of the pigeon-holed wall, which, like the flues, 

 are made to enclose the soil containing the plants ; then, by keeping a 

 part of these warm surfaces generally covered with soil, or with 

 boards, or with any other material which shall operate as a non- 

 conductor, when extra heat is wanted unexpectedly, all that is neces- 

 sary is to take off the non-conducting covers. Even in the case of a 

 common hotbed, heated only by the bed of dung beneath the plants, 

 extra heat may be provided for by bedding a plate of stone, slate, 

 zinc, or cast iron, on the dung, in one or more places of the interior 

 of the frame, according to its size, and covering these with boards, 

 supported at the height of two or three inches above them, so as to 

 enclose a stratum of air, to act as a non-conductor ; the sides being 

 closed by a rim previously formed of cement, or brick-on-edge, on the 

 stone or slate, or by a rim two or three inches deep, cast on the edges 

 of the iron. By taking off the wooden covers, an extra supply of dry 

 heat will immediately be obtained, which may be rendered moist at 

 pleasure by pouring on water. Another mode of obtaining an imme- 

 diate extra supply of heat from a dung-bed is, by bedding in it, when 

 first made, an iron pipe of three or four inches in diameter, with the 

 two extremities turned 

 up, and covered by 

 flower-pot saucers. The 

 length of the tube may 

 b? nearly equal to that 

 of the bed, and the one 

 end must be sunk a few 

 inches deeper than the 

 other, as in fig. 134. 

 It is evident that by 

 taking off the covers of 

 this pipe there will be a 



Section of a dung-bed, with a tube for supplying 

 hot air. 



draught created in it, in consequence of its sides being heated by the 

 dung; and an extra degree of heat will by this means be brought into 

 the atmosphere of the bed. This plan might also be adopted for put- 

 ting the air of a plant-bed in motion, without the admission of the 

 external air. 



Fermenting Materials and Fire-Heat Combined. In pits and low 

 forcing-houses heated chiefly by dung, provision is frequently made for 

 the supply of extra heat, by the addition of smoke-flues or hot-water 

 pipes. Fig. 135 is a perspective elevation and section of a house, in 



M2 



