PLANTS, WITH GLASS ROOFS. 



215 



and late vineries, during damp weather in autumn. It is also necessary to 

 obtain dry heat to ripen the wood of all forced plants ; and, though I have 

 no experience of pines, I do not imagine they will ripen, to be good for anything, 

 except at a high temperature and in a pretty dry atmosphere. In all these 

 cases, then, it is absolutely necessary to prevent the escape of moisture from 

 the troughs. If this can be done, the only remaining objections are the diffi- 

 culty and inconvenience of obtaining a perfect level for the troughs." {Gard. 

 Mag. 1841, p. 152.) Where the level system of heating can be adopted, 

 open gutters would appear to be preferable to closed pipes, as rendering more 

 certain the supply of moisture to the atmosphere of the house, and super- 

 seding entirely the use of cisterns, except in botanic stoves for growing 

 aquatic plants. 



508. Retaining Heat by Coverings. — Whatever mode of heating plant- 

 structures may be adopted, it should be constantly borne in mind that it is 

 incomparably better for the health of the plants to prevent heat from escap- 

 ing by non-conducting coverings during nights, than to allow it to be con- 

 tinually given off into the atmosphere, and as continually supplied by fire- 

 flues or hot-^vater pipes. Where coverings cannot be applied, and a high 

 temperature must be kept up, reserve sources of heat, and abundant supplies 

 of water to maintain atmospheric moisture, are the only means by which 

 the plants can be kept healthy. " A weakly growth," Mr. Paxton observes, 

 " is the sure consequence of a high temperature maintained by fire-heat, 

 whatever plan of artificial heating be adopted." He therefore recommends, 

 in all cases where practicable, the use of external coverings, by which, at 

 Chats worth, a difference of from 10® to 15° is gained, and two thirds of the 

 fuel that would otherwise be necessary are saved. (Gard. Chiton. Jan. 16, 

 p. 40.) 



509. Atmospheric Moisture. — The necessity of proportioning moisture to 

 temperature, and the causes which render the climates of our plant-structures 

 unnaturally dry, have already been pointed out (251 to 257). To give an 

 idea of the quantity of moisture requisite for an atmosphere at a high tem- 

 perature, Mr. Rogers has shown that a vinery twenty-five feet long by thirteen 

 feet six inches wide in the roof, maintained at 65° when the outer au^ is 35°, 

 vtill condense on the glass, in twenty-four hours, 35^ gallons of water. (Gard. 

 Mag. 1840, p. 282.) In devising the best method of procuring a constant 

 supply of moisture for the air of a hothouse proportionable to the expendi- 

 ture, Mr. Rogers finds the end may be most effectually attained by placing 

 cisterns on the heating-pipes. As the temperature of the water in these 

 cisterns would vary with tbat of the pipes, the evaporation from them w^ould 

 be greatest when the pipes were hottest ; when the greatest degree of arti- 

 ficial temperature was being obtained, and consequently the greatest di-ain upon 

 it by condensation. The cisterns may be made of zinc, with their bottoms 

 fitted to the curvature of the pipes, at least six inches deep to the top of the 

 pipes, and of the same lengths as the space between the rings by which the 

 pipes are joined. Where two pipes are placed side by side on the same 

 level, the form shown in fig. 153 may be adopted, and a single pipe may 

 have cisterns fitted to it in the same manner, or it may be made to embrace 

 the sides of the pipe and cover it entirely with water, as in fig. 154. In 

 some cases shallow cisterns are cast on the pipes, but their power is insuffi- 

 cient, and in general zinc cisterns may be considered the best. Cisterns so 

 placed on pipes heated to 200° will contain water at 140° to 145° ; but this 



