J82 FIXED STRUCTURES FOR GROWING 



the sides. In summer, when the temperature of the outer air is as 

 high as that of the house ought to be, openings may be made in every 

 direction at pleasure. In stoves, the precaution of covering the 

 openings of the upper part of the roof, by which air is given, with 

 wire netting might be taken, which, while it excludes wasps and flies 

 in summer, would in winter act like Jeffrey's Eespirator in abstracting 

 the heat from the heated air which escaped, and imparting it to the 

 cold air which entered, or the double tube, recommended by Dr. 

 Arnott in his ' Treatise on Warming and Ventilating,' might be 

 adopted. The external air may be heated in the winter season before 

 it is allowed to enter the house, by enclosing a part of the pipes or 

 smoke-flues in a trunk or box, with a communication at the lower 

 part of one end with the open air, and at the upper part of the other 

 with the air of the house. So long as the pipes are kept at a tempera- 

 ture considerably above that of the house, fresh air will flow in, and a 

 corresponding quantity will be displaced by the accidental crevices of 

 the roof. In hotbeds it is customary to leave openings for the escape 

 of moist vapour during the whole of the night ; this is generally done 

 by raising the sashes behind ; but, as by this mode the steam from the 

 dung is sometimes driven in, some gardeners have a narrow opening 

 in the upper part of the sash, with a lid to fit to it, hinged along the 

 upper edge. 



Light is one of the elements of culture as essential as heat. The 

 health, strength, and beauty of plants, depend very much upon the 

 amount of light to which they are subjected ; hence the immense 

 importance of cheap and good glass to horticulture. With heavy roofs 

 of wood, and panes of glass five inches by three, all lapped, nearly a 

 half of the old frames and roofs were opaque. With the abolition of 

 the duty, these panes gave place to others a yard long by nearly half 

 a yard wide. The results at first appeared somewhat disappointing. 

 The plants were tender and the glass was uneven. Each inequality 

 became a lens to burn the emaciated foliage, and large panes were 

 for a time in bad repute ; but again they are in the ascendant. It is 

 found that with an even surface and healthy plants, burning is reduced 

 to a minimum. The glare of the sun is broken by temporary shading 

 or by the use of tinted glass, as in the great conservatory and new 

 palm-house at Kew. Crown or sheet glass in large squares is the most 

 generally used. This is occasionally butted, or placed end to end, on 

 steep roofs, but it is more generally lapped. The laps should not 

 exceed one-eighth of an inch. The broader the lap, the more liable is 

 the glass to breakage, when the water so retained becomes frozen. This 

 lap is sometimes entirely, and sometimes partially, rendered air and 

 water tight by putty. In the former case it prevents the water which 

 condenses on the inside of the glass from escaping to the outside ; and 

 in the latter, while it allows the condensed water to escape, it also 

 retains, by the attraction of cohesion, as much as fills the space 

 between the lap ; and this water, in severe weather, is apt to freeze, 

 and by its expansion when undergoing that operation, the glass is 

 broken. By having the laps unputtied, not only is there great danger 



