512 PRACTICE OF GARDENING. Part III. 



an angle of fifteen degrees, then forming on it the dung-bed, by which means its surface 

 will be at the same angle as the base ; and, lastly, he constructs the frame equally high, 

 both in front and behind, and placing it on the dung, still retains the above an<rie. 

 {fig. 375.) 



2687. The common form of the mushroom-house and that recommended by Oldacre 

 have been described. (1694. and 1695.) The latter plan, though adopted in several places, 

 does not appear to be so generally countenanced by practical, and especially by market- 

 gardeners, as to justify our giving it a preference in this part of our work. In the 

 greater number of cases where mushrooms are grown for the London market, they are 

 raised in the open air on dung-ridges ; and a number of gentlemen's gardeners make use 

 of back sheds, either closed, or open, and some of old cucumber-beds. 



Sect. VII. Details in the Construction of Culinary Hot-houses. 



2688. There are certain details of construction in glazed structures, on which from their 

 novelty or rarity there is considerable difference of opinion among gardeners. These are 

 chiefly metallic roofs, steam, furnaces, flues, trellises, and ventilators. 



2689. Materials of the roof In the construction of the roof, iron and copper, and 

 other metals, have been lately introduced, in order to admit more light, and be more 

 durable. This improvement, Abercrombie observes, " is at present too new to afford 

 ground for a decisive opinion ;" and Nicol says, " On account of the high price of tim- 

 ber, some are now constructing the framing of hot-houses of cast-iron. I would beg 

 leave to remind such, that there is nothing so prejudicial to vegetation as the dripping of 

 rusted iron ; and would advise, that the frames be well and frequently painted, in order 

 to prevent the bad effects of irony water falling on the foliage and fruit. I am of 

 opinion, however, that iron-framed hot-houses will soon get out of fashion. From the 

 quantity of water that must be used, in order to keep the plants in health, the frames 

 must be often moistened, and will corrode." Not only cast-iron rafters, but roofs entirely 

 of iron have wonderfully increased since Nicol's time. 



2690. Themocle of heating by steam is becoming very general in the neighborhood of the 

 metropolis, and especially by such commercial gardeners as have extensive forcing depart- 

 ments, asLoddiges, Gunter, Grange, Andrews, Wilmot, &c. and wherever there is a range 

 of any extent, this mode seems far preferable to heating by smoke-flues. Nicol gives no 

 opinion on this point ; but M'Phail says, " At present, I must freely own, that I have 

 some doubts both of the cheapness, and superiority in other respects, of this new scheme 

 of forcing by the influence of hot water, over the generally adopted methods of the in- 

 fluence of fire, dung, and tan heat." Even " if found to answer better than fire alone, 

 which I much doubt, it will only, I apprehend, be adopted in gardens where there is 

 much forcing, and therefore, of course, the more simple methods of forcing by fire, dung, 

 and tan heat, will be continued in moderate-sized gardens and in small ones." (Gard. 

 Rem. p. 122.) Experience confirms the propriety of these remarks. 



2691. The furnace used by Nicol is simply an oven, capable of containing less or 

 more fuel, according to the kind of hot-house to which it may be attached, and the kind 

 of fuel to be used, with a grate in front, just large enough to kindle the mass of fuel, 

 and keep it alive. In one of a middle size, the oven is thirty inches long and twenty 

 inches wide ; the grate eighteen inches long and ten broad ; the furnace-door ten inches 

 square ; the ash-pit door ten inches wide but fifteen inches deep, both with circular 

 valves in their centres. The grate is placed close to the furnace-door. {Kal. p. 280. ) Others 

 have been tried, but none answer better for the general purposes of flued hot-houses. 



2692. Flues. Nicol gives the decided preference to flues constructed of brick and 

 tiles, thus — " The sole of two-inch thick tiles, each fifteen inches long, by twelve broad ; 

 jointed on cross bricks on edge, or pillarets, to keep them about four inches clear of the 

 surface. The walls of well-moulded, or stock bricks, six inches clear of each other, and 

 the height of two bricks placed on edge, covered with inch and half thick tiles, each 

 twelve inches long and ten broad, laid the length to the run of the flue, by which means 

 the covers will not be flush with the sides of the flue, but each edge will he champhered 

 or bevelled, which makes the flue look very light and neat. The open or void of the 

 flue will thus be (with the height of two bricks on edge, and two joints of lime,) ten by 

 six inches, or thereby. It is clear, and detached on all the four sides, except the in- 

 terruptions of the pillarets ; and is the most effectual flue of many different sizes I have 

 tried." — Of air flues, the same author observes, " I think I have ascertained the use- 

 lessness of air-flues." Our opinion is that air-flues in most cases are more injurious 

 than useful, and we believe there has been no mode yet discovered for issuing a current 

 of heated air into a hot-house that is not liable to the most decisive objection on account 

 of the risk of heating to excess. A mode of heating air by steam and then intro- 

 ducing it to the house is now disseminating by some London tradesmen under the name 

 of caloriferes, and which is particularly obnoxious to these objections. 



2693. Trellising. " Roof-trellising," Nicol observes, " is now universally of wire, 



