By Sir George Stewart Mackenzie, Bart. 



perpendicular to the plane of glass. Mr. Knight, and the 

 Rev. Mr. Wilkinson, have differed in regard to the proper 

 inclination for the glass ; so that without the rays ever falling 

 perpendicularly upon it oftener than twice in the year, they 

 shall do so, as nearly as possible, during those periods when 

 the influence of the sun is chiefly desired ! It is not my 

 intention to interfere with that question. It will be suffi- 

 cient for me to point out what is the best, and what is the 

 worst form for the glass, leaving it to horticulturists to 

 choose any intermediate degree of perfection, which may suit 

 their purposes. If a form for the glass can be found, such 

 that the rays will be perpendicular to some part of it, during 

 the whole period of the sun's shining, not twice, but every day 

 in the year, that form must be considered as the ne plus ultra. 



It must already have occurred to you, that that form is 

 to be found in the sphere, and it is the segment of a globe 

 which I propose for the glass, when it is desired to receive 

 into a forcing-house the greatest possible quantity of light. 

 The segment is one-fourth, or a semi-dome ; which I consider 

 sufficient, though to catch the sun at all times, during 

 summer, the segment would have to correspond with the 

 greatest segment of the circle which the sun describes. 



That form which deviates from the sphere to the greatest 

 extent, is evidently the worst ; and we may consider the 

 limit to be a plane, perpendicular to the horizon. The ordi- 

 nary form, that of a plane, inclined to the horizon, is better : 

 then comes a polygonal shape; and the nearer this ap- 

 proaches to the sphere, that is, the greater the number of 

 faces of which it is composed, it will be the better adapted 

 to the purpose in view. As it would be attended with great 



