160 FIXED STRUCTURES FOR GROWING 



have any effect on the glass between them. To guard against all risk 

 of breakage from this cause, however, it is only necessary not to fit in 

 the panes too tightly. Indeed, the objection may now be considered 

 as given up by all experienced hothouse-builders. The liability of 

 iron to conduct away heat in winter, and to attract too much in 

 summer, is also found to be an objection more imaginary than real. 

 It is true that iron, from its being a powerful conductor, is liable to 

 undergo sudden changes of temperature, which must, doubtless, render 

 it less congenial to plants that come in contact with it than wood or 

 brick ; though plants do not appear to suffer when the iron is in small 

 quantities, such as the rods to which vines are attached under rafters, 

 wire trellis-work, &c. ; but when the rafters are of iron, and when 

 plants are trained round the iron pillars used in supporting hothouse 

 roofs, it may readily be conceived that they will be injured by them. 

 This will also be the ca.se, more or less, when tender plants are grown 

 close under the glass in hotbeds or pits covered with iron sashes. 

 Indeed, when we consider the much greater weight of iron sashes 

 than wooden ones, and the constant occasion that there is for moving 

 the sashes of pits and hotbeds, we would recommend them in most 

 cases to be made of wood. The injury done to plants in the open 

 air by iron coming in contact with them, can only take place when the 

 iron is of considerable thickness ; because we do not find it in the case 

 of cast-iron espalier rails, or of dahlias, roses, and other open-air 

 plants tied to iron stakes. In plant-houses it probably takes place 

 after the iron has been highly heated by the sun, and then watered, 

 when the chill produced by evaporation will contract the vessels and 

 chill the juices. The greatest objections that we know to iron roofs 

 are the expense and the difficulty of forming them with sliding sashes 

 which shall not rust in the grooves in which they slide ; but this last 

 objection can be obviated, either by forming the styles and rails, or 

 outer frame of the sash, of wood, and the rafters of iron, or the reverse. 

 In the greater proportion of plant-houses, however, sliding sashes in the 

 roof may be dispensed with, air being admitted during winter through 

 apertures in the upper angle of the house in the back wall, or by rais- 

 ing a hinged sash in the upper part of the roof; and in the hottest 

 weather in summer, by these and the sliding sashes or other openings 

 in front. The whole of the objections to iron roofs are now abolished 

 by an improved mode of glazing, that dispenses with the use of putty, 

 and improved methods of ventilation that supersede the necessity of 

 moveable sashes. 



The following description and illustration of Beard's patent 

 metallic hothouses and sash-bars will exhibit most of the latest 

 improvements in metallic hothouses. The foundation of the whole 

 is the patent sash-bar (fig. 132), for which a is the bar; 6, the covering 

 bars ; c, a white metal cap-nut ; and d, the felt that acts as a buffer 

 at all points between the iron and the glass. Under this arrangement 

 no glass can possibly be broken by the contraction or expansion of the 

 iron, and the iron bar becomes almost as narrow as a wooden one. 

 Air is admitted at top and bottom in the most simple manner, by 



