GENERAL PRINCIPLES. 



133 



the opinion that, for early and for very 

 late forcing, narrow houses with a very 

 steep roof are the best, as they give so 

 small an amount of air to be heated by 

 the solar rays ; and those rays are admitted 

 at a proper angle when the sun is low in 

 the horizon, and above it only for a short 

 time. For general crops to ripen during 

 summer, the case is different ; and there- 

 fore wider houses and roofs at a much 

 lower angle have their admitted advan- 

 tages. This is still more the case when 

 the plants, such as peaches and vines, are 

 trained close under the roof. For plant- 

 houses the case is different, as these 

 require to be of greater breadth, so that 

 the plants may have room to develop 

 themselves on all sides, and be on all 

 sides equally exposed to light and air. 

 There is no doubt a limit as to breadth 

 in this case also, for beyond a certain 

 distance the rays of light and heat cease 

 to be useful. What that distance is, is 

 dependent on circumstances. A certain 

 portion of the light which passes through 

 the glass is refracted — that is, it leaves the 

 transparent medium at a different angle 

 from that at which it fell upon it ; and 

 by this change the light is weakened, so 

 that, at a very short distance from the sur- 

 face of the glass, it becomes dispersed and 

 transfused in the atmosphere, in which 

 state, in hothouses, it has no longer the 

 same power on the vital energies of plants. 



For the angle of elevation, vide art. 

 Angle of Elevation. 



Vineries require to be of a greater 

 breadth than peach-houses, on account 

 of the different habits of the plants. An 

 error generally fallen into is, making the 

 former too narrow when they are intended 

 for a general crop. 



In regard to breadth as well as height, 

 there are thus limits beyond which it 

 would be vain to go, if the health of plants 

 is to be a consideration ; for beyond cer- 

 tain limits the rays of light, after passing 

 through glass, however pure, cease to be 

 useful to them. What these limits are 

 has not been as yet, we believe, accurately 

 determined, though it is desirable that it 

 were so. Light would not reach plants, 

 in a state to be beneficial to them, near 

 the ground in the centre of the transept 

 of the Exhibition glass structure ; nor is 

 it even certain that the large elms would 

 long exist in it, even presuming them to 



be in the vigour of youth instead of the 

 decrepitude of old age. They exist at 

 present, and may do so for a short period, 

 because their foliage is so close to the 

 glass roof. The plants brought in during 

 the Exhibition gave evident proofs of their 

 distance from the glass, and all the art of 

 man could not keep them in anything 

 like healthy appearance. Extreme in 

 width and height together must be avoid- 

 ed, but width with moderate height is 

 quite another affair : for, supposing a glass 

 structure covering 50 or 100 acres, if the 

 height do not exceed 10 or 12 feet, then 

 would plants of 2 or 3 feet in height thrive 

 in it ; and in another whose height did 

 not exceed 4 or 5 feet in height, plants of 

 lower habits might be cultivated. If we 

 exceed those limits, be the form of the 

 structure what it may, then we must 

 furnish it with plants or trees of a pro- 

 portionate height. So great is the influ- 

 ence of light on plants, that even at these 

 moderate limits the plants would be found 

 striving by elongation to reach even nearer 

 the glass. We believe, were a plant put 

 under a structure whose sides were con- 

 structed upon the principle of the slides 

 of a telescope, that as they were drawn 

 upwards the plant would follow to almost 

 any height — striving, in fact, to get to a 

 proper distance from the glass — the light 

 from the sides towards the bottom being 

 useful to the plant only during winter, 

 when the sun is low in the horizon ; dur- 

 ing summer, when it is nearer the per- 

 pendicular, such parts would be of com- 

 paratively little use. In very low houses, 

 say not exceeding 8 feet in height, and 

 where the glass reaches to the ground on 

 all sides, if they are so isolated as to be 

 unshadowed by trees or buildings, their 

 breadth may be increased, because the 

 plants in no part would be farther from 

 the glass than some 5 or 6 feet, or at least 

 they may be so placed as to be so. Such 

 houses, however, would be better adapted 

 for summer than for winter, as at the 

 former season the sun would be high in 

 the horizon, and his slightly oblique rays 

 would pass through the roof from May 

 till September ; and judging from what 

 experience has taught us in the case of 

 peaches and vines, trained at nearly similar 

 distances from the glass, a degree of success 

 might be expected. During winter, little 

 light directly useful to plants would reach 



