A]>MISSION OF LIGHT. 337 



Londorij an angle which corresponds to the 20th of May 

 and 2Xst of July. This would afford four months, from 

 the 20th of April to the 21st of August, during which 

 the angle of incidence af mid-day would not at any 

 time amount to 9°, while the deviation at the winter 

 solstice would be 43-°, and the loss of light from reflex- 

 ion would be little more than -^\J . The Rev. Mr. Wil- 

 kinson-recommends 45°, a pitch extremely suitable for 

 exirly vineries and pine-stoves. In this ease, the mid- 

 summer deviation would be .19°, and the loss ^^^j, and 

 the midwinter deviation 30°, while the loss is nearly 

 the same. From these .statements,'howcvcr, and from 

 an inspection of tlie table - already referred to, it is 

 manifest that mueh- greater exactness has been sought 

 in this matter than is at all necessary. The reduction 

 of the opacity of the roof, arising from the breadth and 

 depth of rafters. and astragals, is of much greater con- 

 sequence. Accordingly, in some glazed houses, parti- 

 cularly those constructed of metallic substances, rafters 

 have been omitted altogether.; but this kind ef struc- 

 ture leads to consider^able difficulties ^n the a<;lmission 

 of air. ^. ' • - 



We- have taken it for granted that the framework is 

 composed of wood ; and if prim^ l^ajtic tirtiber be pro- 

 cured, it will endure for nearly half a -century. But in 

 some cases rafters and sashes made entirely of metal, 

 generally either malleable or cast iron, have been em- 

 ployed ; and in others, a middle course has been steered 

 by adopting wooden mortices and metallic tenons. The 

 great objection to the use of metal for rafters and 

 sashes is, that it is too rapid a conductor. p£ caloric, 

 and too liable to contraction and expansion from the. 

 alternations of heat and cold ; the expansion tending to 



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