152 FIXED STRUCTURES FOR GROWING 



either by the evaporation of surface water, or by N., N.E., or KW. 

 winds. If the surface of the soil is hard and smooth so as to carry off 

 the winter rains and thawing snows, without allowing them to sink 

 into and cool the soil, so much the better. It is seldom, however, that 

 these conditions can be fulfilled to their utmost extent ; because not 

 only such situations are not frequent in nature, but that even where 

 they do exist, the situation for the hothouses is determined by the 

 artificial circumstances connected with the house, offices, and grounds. 

 For ornamental structures the situation chosen is generally some part 

 of the pleasure-ground, or flower-garden, not far from the dwelling- 

 house ; and forcing-houses are generally placed in the kitchen-garden, 

 or in some place intermediate between it and the stable offices ('Sub. 

 Arch, and Landscape Gardener,' p. 412). Wherever the situation may 

 be, the soil and subsoil ought to be rendered perfectly dry by drains 

 so placed as to intercept all subterraneous water, from whatever direc- 

 tion it may come ; and by surface -gutters, or the surfaces of walks, &c., 

 so arranged as to carry off the water of cold rains and thawing snows, 

 without allowing it to sink into and cool the soil. The next point is 

 to produce artificial shelter, by walls or other buildings, and planta- 

 tions or hedges, so placed as to check the winds which blow from cold 

 quarters without obstructing the south and south-east winds, and the 

 morning and evening sun. The amount of heat carried off by winds 

 which are at a lower temperature than the surface they pass over, is 

 great in proportion to the velocity of the wind and the moisture of the 

 surface, and hence the much greater ease with which the temperature 

 of a greenhouse may be kept up when it is placed in a sheltered, 

 rather than in an exposed situation ; for example, in the concave side 

 of a curvilinear wall, rather than against a straight wall. 



The most perfect form of house for the admission of solar light 

 and heat is that of a semi-globe of glass, because to some part of this 

 form the sun's rays will be perpendicular every moment while he 

 shines, and at every time of the year ; and by it a maximum of light 

 will be admitted at those periods when he does not shine ; but this 

 form, excepting under particular circumstances that, for example, in 

 which there was a double glass dome, or in which only a temperature 

 of a few degrees above that of the open air was required to be kept up 

 would occasion too great a loss of heat, either for economy or the 

 health of the plants ; for when heat is rapidly conducted away and 

 rapidly supplied by art, it is found extremely difficult to obtain a suffi- 

 cient degree of atmospheric moisture for healthy vegetation. For these 

 reasons a semi-dome is preferable to a semi-globe, because the glazed 

 side being placed next the sun, the other side may be opaque, so as to 

 reflect back both, heat and light, and it may be made so complete a non- 

 conductor as net to allow the escape of any heat. There is an objec- 

 tion, however, to the general adoption of the semi-dome, because it is 

 found that the rays of light after passing through glass-roofs, lose their 

 influence on the plants within in proportion to their distance from the 

 glass and its clearness. Hence, for general purposes, span-roof or lean- 

 to houses are the best ; and hence also herbaceous plants are grown 



