PLANTS, WITH GLASS ROOFS. 195 



twenty-four hours, under ordinary circumstances, in the event of the supply 

 of heat from the consumption of fuel, or the action of the sun, heing discon- 

 tinued from neglect or accident, or hy cloudy weather. 2. To provide means 

 of speedily increasing the supply of heat, when the sudden lowering of the 

 external temperature, or the action of high cold winds, or a cold humid 

 atmosphere among the plants, requires it. 3. To provide the means, by an 

 adequate surface of flue, or steam, or hot-water pipes, of supplying a suffi- 

 ciency of heat in every house, according to the temperature required, not 

 merely under the ordinary external temperature, but when that temperature 

 shall fall as low as 10, or in situations exposed to very high cold winds to 

 zero. 4. To make arrangements for supplying atmospheric moisture in pro- 

 portion to the supply of heat, and for withdrawing this moisture at pleasure. 

 5. Where no means can be provided for supplying extra heat on extraordi- 

 nary occasions, to provide the means of conveniently applying extra external 

 coverings for the same purpose. It is proper to remark, that in every plant- 

 structure there is a reservoir of heat and of moisture, to a certain extent, in 

 the soil in which the plants are grown, whether that soil is in pots or in a 

 bed ; and that all the paths, shelves, and other objects within the structure, 

 being heated to the proper degree, part with their heat, whenever the air of 

 the house falls below the temperature of these objects. This source of heat 

 might be considerably increased in houses where there is abundance of room : 

 for example, below a greenhouse stage, by placing objects there of moderate 

 dimensions and separated from each other ; such as parallel walls of four- 

 inch brick- work, flag-stones set on edge two or three inches apart, or slabs 

 of slate set on edge one inch apart. These, by presenting a great extent of 

 surface, would absorb a powerful reserve of heat, and give it out whenever 

 the other sources of heat were defective. 



489. Fermenting substances, such as stable-dung, tanner's bark, leaves, 

 &c., are either applied in masses or beds under the soil containing the plants, 

 as in the common hotbed ; or in casings or linings exterior to the soil or 

 structure to be heated, as in M'Phail's 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 re- 

 taining 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, is 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 by any other means which 

 shall operate as a non-conductor, when extra heat is wanted unexpectedly, 

 all that is necessary 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 inches 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 



