PLANTS, WITH GLASS ROOFS. 197 



the vines are planted, and which is supported by cast -iron joists and Welsh 

 slates ; and b shows the openings furnished with shutters by which the dung 

 is introduced. Beds on the same plan, but wider, have been used for growing 

 pine-apples and melons, and for various similar purposes. An extra supply 

 of heat from the dung may be obtained by having panels of slate in the inside 

 wall, c, to be kept covered by wooden shutters, except when extra heat is 

 wanted ; or by tubes, as in fig. 134 ; or it may be rendered unnecessary by 

 extra coverings. The first forcing which we read of in the history of British 

 gardening was effected, as Switzer informs us, by placing casings of hot 

 dung against the north side of walls of boards, against the south side of which 

 cherries were trained. 



491. Heating from vaults, or from stacks of flues. The oldest and simplest 

 mode of applying fire-heat to hothouses, was by means of a pit in the floor, 

 or a vault under it. The vault was of the same length and breadth as the 

 floor, with the chimney at one end ; or it occupied a smaller space in the centre 

 of the floor, with a stack of flues rising over it, and forming a mass of heated 

 material in the body of the house. The fire was of wood and made on the 

 floor ; or of charcoal or coal, and made in an open portable iron cage, like that 

 used by plumbers, when soldering joints in the open air, with a plate of iron 

 over it to act as a reverberator, and prevent the heat from rising directly to 

 the roof. The flue by which the smoke escaped had its lower orifice on a 

 level with the floor of the vault, so that the air and smoke did not enter it 

 until they had parted with most of their heat. These modes are capable of 

 great improvement, and in various cases would perhaps be found more eligible 

 and economical than any other, by a gardener who is aware of the importance 

 of connecting with them an efficient means of supplying atmospheric mois- 

 ture : by placing cisterns of water over the hottest part of the floor, or by 

 having dripping fountains formed on the syphon principle, by inserting the 

 ends of strips of woollen cloth in open vessels of water, and placing these in 

 different parts of the house. See on this mode of heating, Mr. Forsyth, in 

 Gard. Mag. for 1841. 



492. Flues. As the mode of heating by vaults could only be adopted when 

 the plants were to be grown in pots or boxes, as soon as the practice of forcing 

 fruit-trees trained against walls, and having their roots in the border or floor 

 of the house, was introduced, flues in the wall against which the trees were 

 trained, and afterwards detached flues along the front of the house, became 

 necessary ; and when these last are properly constructed, and the dry heat 

 which they produce is rendered moist by placing water over them, they form a 

 convenient and economical mode of heating. The flue is always most efficient 

 when carried along the front and ends of the house, because the air imme- 

 diately within these is more liable to be cooled by the external air than that 

 next the back of the house, the back being generally a wall of brick or stone. 

 Where the house is glass on every side, as well as on the roof, the flues will 

 be most efficient if carried round it, for obvious reasons ; while the air imme- 

 diately under the roof, in every case, will be kept sufficiently warm by the 

 natural ascent of the heated air from the flue, in whatever part it may be 

 placed ; though when the flues are placed in the lower part of the house, 

 there will be a greater circulation than when they are elevated ; and this 

 arises from the greater number of particles which must be put in motion by 

 the ascent of warm, and the descent of cold air. The quantity of flue 

 requisite for heating a house to any required temperature has not been 



