PLANTS, WITH GLASS ROOFS. U3 



best in pots in frames ; and were it not for the quantity of glass that 

 would be required, all shrubby and climbing plants would be grown to 

 the highest degree of perfection if trained on trellises parallel to the 

 glass roofing, and at no great distance within it. In pits and frames, 

 herbaceous or low plants are nearer the glass than they can ever be 

 in large houses, in which, unless they are placed on shelves close under 

 the roof they are either at a distance from the glass, as in the body of 

 the house, or they present only one side to it, as when they are placed 

 near the front glass. There is another reason in favour of narrow 

 houses where perfection of growth and economy is an object, which is, 



123. 



Span-roofed forcing-house. 



that the major portion of the heat by which the temperature of hot- 

 houses is maintained, is supplied by the sun. The power of the sun 

 therefore will be great on the atmosphere within, inversely as its cubic 

 contents, compared with the superficial contents of the glass enclosing 

 it. Thus, suppose one house to be twenty feet high and twenty feet 

 wide, and another to be twenty feet high and only ten feet wide, 

 the contents of the former will be exactly double that of the 

 latter ; at the same time, instead of containing double the surface of 

 glass on its roof, it will contain scarcely one-third more ; being nearly 

 in the proportion of twenty-eight for the house of double volume, to 

 not fourteen, or one-half, but twenty-two, for the one of half the 

 internal capacity. In the wide house every square foot of glass has to 

 heat upwards of seven cubic feet of air ; in the narrow house only 



