PLANTS, WITH GLASS ROOFS. 201 



ordinary cold. In country situations, where wood for burning is not very 

 dear, or where coke from coal could be readily obtained, can-flues might be 

 economically employed for drying up the cold damp of greenhouses, and for 

 a variety of purposes. We have said more on the subject of smoke-flues 

 than may be thought necessary at the present time, when they are being 

 so generally substituted by hot -water pipee ; but our object is to prevent our 

 readers from being so completely prejudiced against flues as not to have re- 

 course to them in particular situations and circumstances. The principal 

 reason why so much has been said against smoke-flues is, that gardeners 

 till lately were not fully aware of the importance of supplying moisture to 

 the atmosphere of plant-houses in proportion to the supply of heat, and of 

 having reserve flues, in consequence of which excessive heat would not be- 

 come so frequently requisite, and noxious gases would have less chance of 

 being driven through the top and sides of the flue into the atmosphere of 

 the house. 



496. Steam was the first substitute for flues employed in this country ; 

 and, under some circumstances, it may deserve a preference to either flues 

 or hot water. For example, where the heating apparatus must necessarily 

 be at a great distance from the structure to be heated, steam can be con- 

 ducted to it in a tube not more than an inch or two in diameter, which may 

 be so encased in non-conducting matter as to occasion far less loss of heat 

 than if either smoke or hot water were employed. The disadvantages 

 attending the use of steam in ordinary cases are, the necessity of heating the 

 water to the boiling-point, by which more heat is driven up the chimney 

 and lost than if the water were raised to only half that temperature, and 

 the want of a reservoir of heat when the steam is not in action. The last 

 disadvantage has been supplied by passing the steam-pipes through brick 

 flues filled with stones, through pits or other large masses of stones, or 

 through tubes, cisterns, or tanks of water. By arrangements of this kind, 

 steam can be made both to supply heat permanently and expeditiously. 

 The various details of these modes of heating by steam will be found in 

 the Gard. Mag. vols. viii. and ix. ; and in the Encyc. of Gard. edit. 1832, 

 p. 593. 



497. Hot water is the medium of heating plant-structures now generally 

 adopted, and it is without dispute far preferable to any of the preceding 

 modes. Water is such an excellent carrier of heat, that a house warmed by 

 hot- water pipes is not hotter at one end than at the other, which is almost 

 always the case when smoke-flues are employed : none of the heat which the 

 water derives from the fuel is lost, as in the case of flues, which when coated 

 internally with soot convey a great part of the heat out at the chimney-top ; 

 no sulphureous or other disagreeable effluvium is ever given out by hot-water 

 pipes when they become leaky, as is the case with flues when they are not 

 air-tight ; and the hot water in the pipes serves as a reservoir of heat when the 

 fire goes out ; but smoke-flues, when the fire goes out, are rapidly cooled from 

 within by the current of cold air which necessarily rushes through them 

 till it has reduced the temperature of their tops and sides to that of the 

 open air. Whether heating by hot water is more economical than heating 

 by smoke flues, will depend chiefly on the kind of apparatus employed ; but 

 in general we should say that it was not attended with any advantages of 

 this kind. Mr. Rogers is of opinion that with a well- constructed and well- 

 managed apparatus, the saving of fuel may amount to 25 per cent, over well- 



