PLANTS, WITH GLASS ROOFS. 175 



that by which the heated air escapes. Such a current is formed by 

 the cross drains adopted by Mr. Penn, and exhibited in various 

 sections of plant-structures given in this work. For the same reason 

 it is desirable, when practicable, and under certain circumstances, to 

 confine the pipes on each side, so that the air which passes up among 

 them may not escape without being heated. To illustrate the effect 

 of this arrangement, we may take Perkins' double boiler, and com- 

 pare it with the common boiler. It would not occur to any person 

 who had not reflected on the subject, that water could be boiled any 

 sooner in one boiler than another, both boilers being of the same 

 dimensions, made of the same material, set in the same manner, and 

 with a fire beneath them of the same power. Yet such is the case ; and 

 this exactly on the same principle that we recommend confining the 

 sides of hot-water pipes, and supplying the air to be heated from a 

 distinct channel. Suppose we have a common boiler, such as is used 

 in common wash-houses, then place another boiler within it, of such a 

 size as to leave only a few inches between the inner boiler and the 

 outer boiler all round, and support it in this position by stays, as shown 

 in fig. 146 ; let this inner boiler have a hole in y 



its bottom about one-third of its diameter, and 

 let its rim be two inches below the level of the 

 water to be heated. These arrangements being 

 made, and the heat applied below, a circulation 

 instantly takes place and continues, the water 

 coming into contact with the heated bottom and 

 sides of the outer boiler, rising rapidly to the 



surface, and descending through the inner ^^s double bo t ler. 

 boiler, which thus necessarily contains the coldest portion of the liquid. 

 (* Gard. Mag.,' vol. xvi. p 325.) The heat communicated by the fire 

 to the bottom and sides of the outer boiler is rapidly carried off by the 

 current that is created, exactly on the same principle that wind, which 

 is a current of air, cools any body exposed to it more rapidly than air 

 at the same temperature but quite still. The underground drains 

 should either have vacuities at the sides and over the top to prevent 

 them from absorbing much heat, or they may be carried through the 

 bottom of the tan-pit, where there is one. In general, we would not 

 cover the heating-pipes, nor would we adopt the upright tubes which 

 Mr. Penn originally used, but has since dispensed with. There may be 

 situations and circumstances where it would be more desirable to have 

 the heat of the pipes or flues carried off by radiation with the usual 

 degree of slowness rather than by conduction ; such, for example, as 

 when the attendant on the hothouse was likely be a long time absent, 

 or when some danger from overheating was anticipated ; and this can 

 always be attained by covering the orifices by which the air enters to 

 the cross-drains. It is proper to state, that at the present time the 

 opinions of a number of persons are against the use of air as a carrier 

 of heat in hothouses, on account, they say, of the difficulty of maintain- 

 ing it in exactly the proper state of moisture. This, however, can be 

 effected without difficulty, by keeping the bottoms of the cross-drains 



