176 On the Form of the Glass of a Fwcing-house. 



for the glass, are easily made of cast iron. The distance be- 

 tween them at the base may be about fifteen inches ; and 

 when the gores contract to half that width, every alternate 

 rib may stop, as seen in Fig. 2. 



As it is, at times, desirable to defend plants from the sun, 

 when the heat is too great, two external moveable ribs may 

 be placed on a pivot at the top, and to them may be fastened 

 a gore of canvas, which, when stretched, by keeping the 

 ribs asunder, may cover one-half of the glass ; or several of 

 a smaller size may be used. 



There is one inconvenience (if indeed it be really one) 

 which attends the construction I have proposed, viz. that the 

 plants cannot be exposed to the rays of the sun, without the 

 intervention of the glass. Such an exposure has been found, 

 by Mr. Knight, to be of great use during the time of the 

 ripening of peaches. But it appears to me uncertain whe- 

 ther the advantage be derived from any additional quantity 

 of light. I am rather inclined to think, that the fruit being 

 exposed to the air, the process of ripening is prevented from 

 proceeding so rapidly as it would otherwise do ; and is thus 

 rendered more perfect. I attribute the advantage more to 

 the air than to the light. But this is a question which may be 

 easily determined by experiment; and I hope Mr. Knight will 

 put it to that test. Whether an equal advantage be derived 

 from exposure by grapes, my experience does not allow me 

 to say ; though I believe, that all kinds of fruit should have 

 as much fresh air as possible during the time of ripening. 

 I have the honour to be, Dear Sir, 



your obedient and faithful Servant, 



George Stewart Mackenzie. 



•'•nburgh, V,th July, 1815. 



