1869.] PORTABLE AND CONVERTIBLE GLASS SCREENS. 279 



in a popular sense), and it is just here that such assistance might readily be given, 

 as to bring the former off an easy victor. 



We have glanced at the strong points of glass protectors, let us now look for 

 a moment at their one weak place. Transparency is their chief merit, but it is 

 likewise their greatest demerit. It is a law of transparent bodies in general, and 

 of glass in particular, that heat can be spirited away through them by what is 

 termed radiation, — that is, nature's perpetual struggle after an equilibrium of 

 temperature. There are two obvious methods of counteracting this force of 

 radiation. One is by the creation of an internal source of heat, to glut to satia- 

 tion this greedy robber of caloric, and yet generate a sufficient reserve for pro- 

 tective purposes ; but this mode is expensive. Another is to baffle the thief on 

 his own ground, by hiding the transparent face of glass by an opaque covering. 

 The thinnest bunting or canvas spread over the outside — or, better still, the 

 inside — of glass, would neutralize the powers of radiation, and it is surprising 

 that this check is not more generally used. 



But ready portability is one of the first characteristics of all good wall tree- 

 protectors. Their value may be said to sink or rise in proportion as they are 

 portable or otherwise. Fixed protectors induce tenderness, and thus, while they 

 work to exclude the cold, they render the tree susceptible of equal injury by a 

 less amount of it. Hence the common saying, that protection does as much or 

 more harm than good. Hence, also, the various modes, by pulleys, cords, rings, 

 rollers, &c, of imparting ready motion to canvas or blinds used against walls. 



With glass protectors the importance of portability has been less thought 

 about, because its necessity is less obvious. They do not exclude the light, and 

 light is the best antidote to weakness. But in these days, when the economy of 

 force is better understood, and it has been discovered that no force is expended 

 in vain in the economy of the universe, it is, to say the least of it, a reckless 

 waste of power to erect a permanent glass screen to effect a purpose which would 

 be equally well or better accomplished by a temporary shelter. 



Hitherto, one of the chief difficulties in the way of using temporary glass screens 

 has been our manner of building and glazing glass houses. Glass, as we all know, 

 till recently has been brittle in quality and expensive in price. We have thus 

 become almost afraid to touch it. When employing it, our object has been to get it 

 out of hand and beyond risk of breakage as soon as possible, and to leave it alone 

 as long as we could. Hence our monstrously cumbersome and solid modes of build- 

 ing, and our permanent, immovable methods of glazing. They were bom of the 

 dearth, inferiority, and high price of glass, but the time is at hand when horti- 

 culturists will use house glass with as much ease and as little risk or fear of 

 breakage as they now use garden pots. The abolition, by Mr. Beard, of sticky 

 modes of glazing, has been one great step in this direction. The doing away with 

 all glazing whatever, as Mr. Bendle has done in his portable tile and glass 

 protectors for the growth of fruits, flowers, or vegetables, is another. These 



