138 



HOTHOUSE-BUILDING. 



are perpendicular, as appears by Bouguer's 

 table of rays reflected from glass. 



"Of one thousand incidental rays, when 

 the angle of incidence is 



87° 30', 584 are reflected. 



85°, 543 



82° 30', 474 



80°, 412 



77° 30', 356 



75°, 299 



70°, 222 



65°, 157 ) 



60°, 112 

 50°, 57 ~ 

 40°, 34 

 30°, 27 ~ 

 20°, 25 

 10°, 25 „ 

 1°, 25 ~ 



—Hort. Trans, ii., p. 227. 



" Now, if we suppose a roof," says Mr 

 Loudon, "in one plane, with the sun 

 shining on it at six o'clock in the morn- 

 ing, and at six o'clock in the afternoon, 

 at an angle of 85 c , which would be the 

 case in March and September, fully one- 

 half of the rays which fell on the roof 

 would be reflected ; while in the case of 

 a ridge-and-furrow roof, if it shone on 

 half the roof — that is, on one-half of each 

 of the ridges— at any angle, with a perpen- 

 dicular not exceeding 30°, at the same 

 periods only two per cent of the rays 

 would be reflected. Suppose, then, the area 

 of the entire roof, taken as one plane, to 

 be 100 square yards, and, to facilitate cal- 

 culation, that only 100 rays fell on each 

 yard, then the total number which would 

 enter through the roof in one plane would 

 be 50,000, while those that would enter 

 through the ridge-and-furrow roof would 

 be 99,000, or very nearly double the 

 number." — Suburban Horticulturist. 



The majority of gardeners and hot- 

 house builders appear to agree in fixing 

 upon 45° as being the angle for hothouse 

 roofs for general purposes, not only as 

 being that at which the rain water will 

 drain most freely from it, but, according 

 to these views, as being that most favour- 

 able for plants in general. 



In regard to the angle of roofs losing 

 heat by radiation and conduction in pro- 

 portion to their height, we have the fol- 

 lowing reasoning by Mr Perceval in 

 the " Gardeners' Chronicle :" — " The loss 

 by radiation," he says, " must be the same 

 at all angles ; but the cooling effect of the 

 wind will be greater on a high-pitched 



roof than on one that is flatter, though 

 I believe the difierence in this respect 

 (except inasmuch as it increases the 

 length of the rafter, and, therefore, the 

 cooling surface of the glass,) will gene- 

 rally be too small to be of any practical 

 importance. The effect appears to me to 

 arise in this manner : the wind being 

 supposed to move horizontally, the force 

 with which it strikes the glass will be as 

 the sine of the angle which the roof forms 

 with the horizon ; and the greater the 

 force of the blow, the more particles will 

 there be brought into contact with the 

 glass ; and this effect will be the same as 

 that of an increased velocity of the wind. 

 On this latter subject Mr Hood has given 

 some interesting and ingenious illustra- 

 tions in his book ; and I think his rules 

 for the cooling effect of wind at dif- 

 ferent velocities will apply to this ques- 

 tion. He has shown the cooling effect of 

 wind on glass to be as the square root of 

 the velocity — or that the velocity must 

 be increased four times to produce twice 

 the cooling effect. Now, the force of a 

 blow being as the square root of the velo^ 

 city of the striking body, and as the sine 

 of the angle of incidence conjointly, we 

 shall find that, if two glass roofs be seve- 

 rally pitched at 30° and 40°, the force of 

 the wind upon them, at equal velocities, 

 will be simply as the sines of these angles, 

 or as 50 is to 64 : this will be equivalent 

 to increasing the velocity of the wind 

 from 1.0 to 1.28, and the cooling effect 

 will therefore be as the square root of 

 these numbers, or as 1.0 is to 1.13. But 

 this will only apply to that part of the 

 heat given off by conduction — the radi- 

 ation not being affected by this cause. 

 And Mr Hood has shown, in his book, 

 that at common temperatures the loss by 

 radiation from glass is nearly two-thirds 

 of the whole effect ; therefore the loss, by 

 increasing the angle of the roof 10°, will 

 be one-third of the above amount, or 

 about one-thirtieth more than a flatter 

 roof would be. This effect, of course, is 

 only on the roof, all the rest of the house 

 being entirely unaffected by this cause ; 

 therefore we are justified in concluding, 

 that the pitch of the roof makes but little 

 difference in the cooling effect, except by 

 the increased length of the rafter." 



For Speechly's opinion of the angle of 

 elevation, vide Vineries. 



