REFERENCE TO HORTICULTURE. 



91 



of plants. We are not aware that the cause of the inefficiency of light, 

 after it has passed through glass and reached a certain distance, has been 

 fully explained ; hut the fact is well known to gardeners, who, in hothouses, 

 invariably place the plants they wish to thrive best at the shortest distance 

 from the glass. As the quantity of light which passes through glass at the 

 roof of hothouses is, all other circumstances being the same, greatest when 

 the plane of the roof is at right angles to the plane of the sun's rays ; 

 hence, the slope of the roof is, or ought to be, adjusted to the direction of the 

 sun's rays at that season of the year when its light is most wanted. As in 

 houses for early forcing, the greatest deficiency of solar light is in the winter 

 season, when the sun is low, so the roofs of such houses are made steep, in 

 order that the sun's rays may be received at a larger angle. Summer forcing 

 houses, on the other hand, have less steep roofs, so as to receive most benefit 

 from the sun in April, May, and June, when forced fruits are ripening. A 

 greenhouse, in which no fruit is ripened, but in which abundance of light 

 is required all the year, has commonly perpendicular glass to receive a maxi- 

 mum of light during winter ; and a sloping roof of glass at an angle of 45°; 

 which is found favourable for the admission of light at every season, as well 

 as for throwing off rain, &c. This subject, however, will receive more 

 attention when we come to treat of the construction of hothouses, 



282. The light of the sun, after it has passed through the clouds, is 

 refracted^ to a certain extent, in the same manner as when it passes through 

 glass or water; and if plants were kept constantly under a cloud, but at 

 some distance from it, and if the space in which they grew were enclosed 

 by clouds on every side, we believe the effect on the plants thus enclosed 

 would not be materially different from that produced by an enclosure of 

 glass. In the open air, however, clouds are not stationary ; and even where 

 a succession of clouds covers growing plants for several days together, the 

 space on which the plants grow is open on every side for the access of re- 

 flected and transfused light. This prevents the etiolation and want of colour 

 which are found in plants in the back parts of hothouses having shed-roofs ; 

 but which are never found in nature, even on the north side of walls, except to 

 a very small extent. Hence plant structures M'hich are enclosed by glass on 

 every side, and which are circular in the plan, are more likely to produce 

 an equalization in the growth and appearance of the plants within, than 

 such as have glass on one side, and a wall or opaque body on the other. 



283. As an isolated body, such as a cone or small hill, dkpei'ses light 

 most extensively when the sun shines, so when the sun is obscured by clouds 

 the same body receives most of the reflected light transfused in the atmo- 

 sphere, because it is exposed to the atmosphere on every side. For the 

 same reason the summits of all bodies in the free atmosphere receive more 

 light than their sides ; and hence the trees in dense forests, and the thickly- 

 standing corn plants in cultivated fields, continue to grow and thrive though 

 they receive little benefit from light, except from that which strikes on the 

 tops of the plants. Hence the great importance of perpendicular light to 

 plants under glass, and the advantages of conical, domical, angular, or 

 ridge and furrow roofs, to plant structures ; because they receive from the 

 atmosphere the transfused light on every side. Hence also, if only a certain 

 quantity of glass were to be allowed for the construction of a plant house, 

 the most beneficial application of it would be in the roof. In the construction 

 of conservatories about sixty years ago, it was customary to have opaque 



