J88 FIXED STRUCTURES FOR GROWING 



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. 



482. The Form. The most perfect form 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 (281) ; but this form excepting under parti- 

 cular 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 

 sufficient degree of atmospheric moisture for healthy vegetation (267 to 271 ) . 

 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-con- 

 ductor as not to allow the escape of any heat. There is an objection, however, 

 to the general adoption of the semi-dome, because it is found (281) 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. Hence for 

 general purposes a long narrow house is the best ; and hence also herbaceous 

 plants are grown best hi pots in frames ; and were it not for the quantity of 

 glass that would be required, all shrubby and climbing plants would be 

 grown to the highest degree of perfection if trained on trellises parallel to the 

 glass roofing, and at no great distance within it. In pits and frames, herba- 

 ceous or low plants are nearer the glass than they can ever be in large houses, 

 in which, unless they are placed on shelves close under the roof, they are 

 either at a distance from the glass, as in the body of the house, or they pre- 

 sent only one side to it, as when they are placed near the front glass. 

 There is another reason in favour of narrow houses where perfection of 

 growth and economy is an object, which is, that a considerable portion of the 

 heat by which the temperature of hothouses is maintained, is supplied by 

 the sun. The power of the sun therefore will be great on the atmosphere 

 within, inversely as its cubic contents, compared with the superficial con- 

 tents of the glass enclosing it. Thus, suppose one house to be twenty feet 

 high and twenty feet wide, and another to be twenty feet high and only ten 

 feet wide, the contents of the former will be exactly double that of the 

 latter ; at the same time, instead of containing double the surface of glass on 

 its roof, it will contain scarcely one third more ; being nearly in the propor- 

 tion of twenty-eight for the house of double volume, to not fourteen, or 

 one half, but twenty-two, for the one of half the internal capacity. In 

 the wide house every square foot of glass has to heat upwards of seven 

 cubic feet of air ; in the narrow house only about four and a half 

 feet (Gardeners Magazine, volume xiii. page 15). There are, however, 

 plant-houses erected not merely for growing plants, but for walking into 

 hi order to enjoy them ; and in these, other considerations interfere with 



