62 STRUCTURES ADAPTED TO PARTICULAR PURPOSES. 



A double-roofed house can be erected on the same plan, by 

 substituting a row of columns along the centre of the house for 

 the support of the ridge, in place of the back wall ; one of the 

 planes being raised the necessary height at each end, for the 

 doors, which must also be done in the single roof, (Fig. 19,) 

 unless the door enters through the back wall, which, in some 

 cases, may not be so convenient as having them at the ends, 

 though, for the economizing of glass surface, we would prefer 

 them in the back wall. 



Although double-roofed houses are generally of a rectangular 

 shape, yet they admit of every combination of form without 

 militating against the admission of light and air. Nevertheless, 

 that they may be perfectly adapted to the end in view, there are 

 rules to be observed, and errors to be guarded against, which it 

 is necessary here to point out. 



If the house is above fifteen feet in width, it is necessary to 

 have a single or double row of columns in the centre to support 

 the ridge of the roof, but in many houses these columns are 

 three times thicker and heavier than they ought to be, even with 

 a due regard to strength and durability. When the columns 

 are disproportionately heavy, the house has a dull and clumsy 

 appearance, and the effect within is extremely bad. Indeed, 

 columns ought to be dispensed with where they can possibly be 

 spared, consistent with strength in the structure. We have 

 frequently seen the internal view of double-roofed houses com- 

 pletely spoiled by the clumsiness of the columns supporting the 

 roof, even when columns were altogether unnecessary. Cast- 

 iron columns are always preferable to timber, even when the 

 structure is made of the latter material. When the colunms or 

 rafters are bound together by braces and crossbars of slight con- 

 struction, as of iron in different forms, vines and other climbing 

 plants may be trained upon them, and be hung in festoons from 

 column to column, or otherwise, as fancy may dictate; this 

 gives an elegant appearance, and is always pleasing to the spec- 

 tator. 



Another common error in the construction of fruit-houses is, 

 the heaviness and height of the front, something in the fashion 

 of the heavy and dull-looking plant-houses of the last century 



