518 Retrospective Criticism. 



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 com- 

 monly perpendicular glass to receive a maximum 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." 



Plants suffer most at a distance from light, when the light is only from the 

 top, or one-sided. This has been called the attraction of light, but is no 

 explanation. In the one-sided light, it may be the greater solidifying of the 

 side next the light which draws. In the top light of frames, the want of 

 direct light at the sides may cause partly the greater elongation of the top ; 

 but plants elongate below glass, even though surrounded by light. The want 

 of motion is a great cause of this : plants uniformly elongate more in a 

 sheltered than an exposed field. If there is any such thing as attraction 

 between light and plants, as roots follow their food (which is partly hygro- 

 scopical in the latter case), it will be, like the attraction of gravitation, more 

 easily perceived in its effects than capable of explanation. Refraction will 

 disperse the light ; it is difficult to understand how it should weaken what 

 does pass through. The chemical power of light, however, is so much con- 

 nected with electricity, that it may be weakened in a way we cannot account 

 for. The chemical power of light is greatest in the least luminous part of 

 the rays ; and yet, as the quantity of light is equal, that of the equator must 

 have most power. There is a connexion between heat, light, and electricity, 

 not yet explained ; the optical qualities of light have been much more at- 

 tended to than the chemical. The red rays have more momentum than the 

 blue ; thus causing the red of the rising and setting sun, and the azure blue 

 of the sky. Perhaps more of the blue, or chemical portion of the sun's rays, 

 may thus be lost in refraction. 



" 463. Canvass coverings for glazed structures or detached plants require, 

 for the most part, to be in framed panels, as well to keep them tight as to 

 throw off the rain, and to prevent them from being blown and beat about by 

 the wind. To render the canvass more durable, it may be oiled, tanned, or 

 soaked in Kyan's or in Burnett's anti-dry-rot composition. When applied to 

 cover the glass sashes of frames or pits, it should be in panels in wooden 

 frames of the size of the sashes j and this is also a convenient and safe mode 

 of forming temporary structures for protecting standard plants or trees : but 

 by suitable arrangements, to be hereafter described, canvass or netting for 

 protecting walls may be hooked on and fastened without wooden frames. 

 This is done in a very efficient manner in the garden of the Horticultural 

 Society of London, to protect a peach-wall. The stone coping of this wall 

 projects over it about an inch and a half, with a groove or throating under- 

 neath. Coping-boards 9 in. broad, fitted to join at their ends by means 

 of plates of iron, are supported on iron brackets built into the wall. The 

 upper edge of the board is slightly beveled, so as to fit as closely as possible 

 to the under side of the coping of the wall, in order effectually to obstruct 

 the radiation of heat and the ascent of warm air. From this coping, woollen 

 netting of various kinds, common netting such as fishermen use, bunting, and 

 thin canvass, have been let down, and tried experimentally, in the course 

 of the last fifteen years ; and we are informed by Mr. Thompson, that, after 

 repeated trials, the thin canvass was found the preferable article for utility, 

 appearance, and duration. This description of fabric costs about 4rf. per 

 yard, procured from Dundee, &c." 



I should think any protection from frost would be much more effectual, 

 if drawn up or removed during a mild day ; the plant would be hardier also, 



