GENERAL PRINCIPLES. 



125 



Fig. 128 shows the mode of training the 

 vine or the peach under such roofs. 



That forcing-houses glazed all round, 

 whether curvilinear or span-roofed, will 

 eventually come into general use, there 

 can be little doubt, the more especially as 

 glass is now so much reduced in price, 

 and also because the advantages of such 

 houses are becoming daily more evident. 

 Our former objection to them was their 

 great expense when compared with lean-to 

 houses; but that objection is now re- 

 moved, as well as that which was founded 

 on their insufficient ventilation, more 

 complete modes of ventilation having 

 been discovered. 



Of all forms of curvilinear structures, 

 that of the semi-globe is the best calcu- 

 lated for the admission of solar light and 

 heat, because, on account of its form, the 

 sun's rays will be perpendicular to it 

 every moment that it shines, and at every 

 time of the year; and by it the greatest 

 amount of light will be admitted when it 

 does not shine. It is, however, the worst 

 form in which to maintain artificial heat, 

 on account of the great loss of heat 

 through the larger surface of glass. The 

 semi-dome is preferable to the semi-globe, 

 on account of the back side being opaque, 

 and, consequently, reflecting both heat 

 and light. This form is, however, ob- 

 jectionable in several respects; and, in 

 particular, on account of the rays of light 

 losing their influence on the plants after 

 passing through glass roofs, in proportion 

 to the distance of the plants from the 

 glass. 



Metallic hothouses. — Regarding the me- 

 rits or demerits of metallic hothouses, 

 opinions appear to be still as much at 

 variance as they were twenty years ago. 

 Heating and ventilating are now much 

 better understood than they were in those 

 days; and, with this increase of know- 

 ledge, some of the evils of metallic houses 

 have disappeared. On this subject Mr 

 Marnock observes — " It cannot be denied 

 that iron hothouses are more susceptible of 

 external changes of temperature, whether 

 from heat or cold, than such as are built 

 of wood ; but if two houses properly con- 

 structed, and equally and fully provided 

 with the means of ventilation, were to be 

 compared, and the amount of difference 

 in the fluctuation and changes occasioned 

 by the iron roof, as compared with the 



one made of wood, were carefully ascer- 

 tained, it would be found much less than 

 is generally supposed. We ought, how- 

 ever, to state, that in small erections — 

 such as amateurs are in the habit of 

 building — the inconveniences occasioned 

 by the sudden fluctuations of temperature 

 are much sooner felt than in large struc- 

 tures, and still more, of course, under 

 iron than under wood roofs. As these 

 fluctuations depend very greatly on the 

 size of the house, the evil is much aggra- 

 vated when the structure is small, from 

 the fact that the smaller the volume of 

 air contained within, the more speedily is 

 it heated and the change felt. Iron 

 houses of small dimensions do, therefore, 

 require more attention in shading and 

 giving ventilation than houses of the 

 same size with wooden roofs. Metallic roofs 

 have, however, this advantage — in dull 

 weather they admit more light, and in 

 winter this is often important. For 

 large structures we decidedly prefer iron. 

 Where the internal area is extensive, and 

 contains a large volume of air, the tem- 

 perature is, in this case, much less sus- 

 ceptible of change. The form of the roof 

 can also be made more elegant than with 

 wood; and the free admission of light, 

 when the plants become large and shade 

 one another, is of the utmost consequence. 

 For forcing-houses of large dimensions, 

 where the trees — whether vines or 

 peaches — are kept a proper distance from 

 the roof, we also think iron preferable to 

 wood. With regard to the contraction 

 and expansion of metal roofs, we never 

 found this practically of the least impor- 

 tance, either for good or evil, though we 

 think this quite likely to be felt in cases 

 of ill-contrived and improperly fitted up 

 roofs." After enumerating the following 

 metallic houses — viz. the large conserva- 

 tory in the Regent's Park Botanic Gar- 

 den, that in the garden of the Horticul- 

 tural Society at Chiswick, the palm-house 

 at Kew, the ranges at Si on House, Eaton 

 Hall, the Botanic Gardens at Glasnevin 

 and Belfast in Ireland, and several places 

 on the Continent, all exceedingly orna- 

 mental works of design and taste — Mr 

 Marnock concludes by observing, " Still 

 we think it right to repeat, that for small 

 structures attached to dwelling-houses, 

 where an efficient gardener and ample 

 means are not at command, wooden roofs 



