398 



PLANT-HOUSES. 



elevation, and in several ground-plans of 

 portions of this range. The top or ridge 

 ventilation h is the same as in all the 

 other houses. A capacious rain-water 

 tank is placed under the centre plant- 

 table, supplied from the roof, and, in the 

 event of an insufficiency of this, from the 

 general supply which is laid over the 

 garden. 



The merits of span-roofed greenhouses 

 are not entirely confined to the admission 

 of more light and air than lean-to houses ; 

 they possess other and very important 

 advantages, " for," says Mr James 

 M'Nab, (in a communication in the 

 "Edinburgh Advertiser,") "in such a 

 greenhouse fire-heat is scarcely at all 

 required ; for if there be a free circula- 

 tion of air during the autumn and winter 

 months, and if the tables and shelves be 

 carefully kept dry and clean, and water 

 be sparingly given to such plants only as 

 require it, cold, even should it extend to 

 the occasional freezing of the surface-soil 

 of the pots, will do less injury to most 

 plants than the application of fire-heat." 

 We have seen, under this judicious culti- 

 vator's management, in the garden of the 

 Caledonian Horticultural Society, while 

 he presided over that establishment, his 

 experiments testing between the plants 

 in a lean-to house and those in a span- 

 roofed one. In the former, soft and hard 

 wooded plants almost alike became drawn 

 up, soft, and spongy towards autumn, 

 from want of the bracing breezes that 

 those of the same species enjoyed in a 

 span-roofed house within a few yards' 

 distance. A severe frost came on : the 

 plants in both houses, early in the morn- 

 ing, had much the same frozen appear- 

 ance ; "by ten o'clock the sun shone 

 forth. The plants in the lean-to house 

 were subjected to the full influence of the 

 mid-day's rays ; and although air was 

 given, they blackened and perished. In 

 the span-roofed house, extending north 

 and south, the effects of the sun were much 

 less felt ; for as he proceeded towards the 

 meridian, the intercepting astragals and 

 rafters necessarily formed a screen or 

 shade ; and air being given, the plants 

 survived, and soon recovered." Experi- 

 ments since made have shown the follow- 

 ing curious results : — "The self-marking 

 thermometer in the open air, during seve- 

 ral nights, indicated 20°, 15°, and even 10° 



Fahr. During these frosts no heat what- 

 ever was applied to the span-roofed house, 

 which contained a general collection of 

 soft and hard wooded plants." On the 

 mornings of these frosts, " the mercury 

 in the thermometer within the house 

 stood at 25°, or 7° below freezing ; yet 

 only two or three plants, which were 

 standing near the upright glass of the 

 south end of the house, and were thus 

 exposed to the mid-day sun, suffered from 

 the intense cold to which they had been 

 subjected. The temperature in the span- 

 roofed house always remained much more 

 equable than in the lean-to house. This 

 was signally remarkable at one p.m. of 

 the 14th of February, when the thermo- 

 meter in the open air indicated 56°, in 

 the lean-to house 70°, and in the span- 

 roofed house 43°. In the lean-to house, 

 therefore, where the whole glass-roof was 

 fully exposed to the sun's meridian rays, 

 the temperature thus became 14° higher 

 than the open air, and 27° higher than in 

 the span-roofed house." 



Nor is it only in resisting the effects of 

 cold during winter that the merits of 

 span-roofed houses consist ; they act also 

 beneficially during the heat of summer in 

 keeping down the temperature, and from 

 the very same causes, namely, " the freest 

 possible circulation of air, by means of 

 upright sliding sashes on both sides of 

 the house ; while the rafters and astragals 

 of the roof break and intercept the sun's 

 rays, and help to shade the plants from 

 their direct influence." Both these effects, 

 it quite clearly appears, would be consi- 

 derably lost, if large, and particularly 

 broad, squares of glass were employed in 

 the construction, as there would be fewer 

 astragals to break the force of the sun's 

 rays ; and this defect would be increased 

 were rafters entirely dispensed with. It 

 follows, therefore, that after all that has 

 been said about large squares of glass and 

 mere glass cases, to have the advantages 

 above stated, we must keep to the small 

 glass and many astragals. Notwith- 

 standing all this, we are still anxious to 

 see rafters and framed sashes entirely 

 superseded by roofs all in one piece, and 

 the glass, where these results are wished 

 to be carried out, retained at the usual 

 size of from 6 to 8 inches in breadth. 

 Their length is of less importance, al- 

 though a certain loss in the above result 



