34 DESIGN. 



cannot be mistaken, even by the most inattentive, " Give us light, 

 or we shall die ! " he gives up their case, in hopeless despair, as 

 being altogether beyond his control. And thus we have known 

 excellent gardeners censured for neglecting things, and for doing 

 badly what it was not in their power to do better. 



Solar influence being necessarily connected with the roofs of 

 hot-houses, we will discuss these subjects in their relation to 

 each other, including inclination and reflection, in the following 

 sub-section. 



3. Slope of hot-house roofs. — In regard to the theory of the 

 transmission of light through transparent bodies, we have already 

 stated that the influence of the sun's rays on any surface is 

 directly as his perpendicularity is to that surface ; and, accord- 

 ing to Bouquer, that if one thousand rays fall perpendicularly 

 upon a surface of glass, the whole pass through, excepting about 

 twenty-five rays, or one fortieth part of the whole. But falling 

 on the same surface at an incidental angle of about 75°, then 

 two hundred and ninety-nine, or nearly one third of them, will 

 be reflected. The influence of the sun on the roofs of hot-houses 

 depends very much on the principle there given, — at least, so 

 far as regards the form of its surface. This principle has been 

 applied, in various ways, for the purpose of obtaining the full 

 influence of the sun's rays at certain seasons of the year. We 

 have managed forcing-houses where the roof was laid at right 

 angles to the sun's rays in mid-winter, — the period when the 

 most powerful rays were required for forcing purposes. 



Although it cannot be denied that much more depends on the 

 manacrement of the house, for the success of cultivation, than on 

 the inclination of the roof, yet it is the most satisfactory method 

 to proceed on what may be considered something like princi- 

 ples. And in this country we find this the more necessary, 

 because the heat of the sun's rays, at certain seasons of the year, 

 is so violent as to prove injurious to vegetation under any cir- 

 cumstances. And hence, this principle should be adopted in the 

 construction of hot-house roofs, that their perpendicularity to the 

 sun's rays, at the hottest period of the year, should by all means 

 be avoided. 



