FORCING DEPARTMENT. 310 



necessity, be proportionably decreased. In addition 

 to this disadvantage, it must not be forgotten, that, of 

 late years, the atmosphere of Forcing-Houses, &c. 

 is constantly kept infinitely more humid, than for- 

 merly was the case, so that the roofs, being perpe- 

 tually exposed to artificial heats and damps inter- 

 nally, and to the very frequent changes of the 

 weather externally, they are subject to every 

 destructive influence, which must operate more 

 rapidly on the wood, and the injurious effects of 

 which can only be partially stayed by the frequent 

 and expensive application of paints. 



Metallic roofs are, however, represented to be sub- 

 ject, in an extraordinary degree, to contraction and 

 expansion, and, consequently, liable to break much 

 more glass than wooden ones. As regards these 

 shews of objections, I can confidently assert, that I 

 have not yet, during the five years that the houses 

 have been erected, observed one pane of glass 

 broke in the whole range of metallic houses here, 

 either by expansion or contraction ; and further, 

 that, during the severe frost, in the Winter of 

 1829, when the thermometer indicated 29 degrees 

 of frost, not a pane of glass was broke by it in the 

 metallic range, where we had upwards of 200 squares 

 cracked in the range of wood houses. I, however, 

 do not mean to imply that this number of squares 

 was broken in consequence of the houses being con- 

 structed with wood ; it might be, and undoubtedly 

 was, in fact, occasioned by imperfect glazing ; but the 

 fact will show how ridiculous it is to impute a greater 

 breakage of glass to the use of metal materials. In 

 2 s 2 



