PLANTS, WITH GLASS ROOFS. 159 



Now, if we suppose a roof in one plane with the sun shining on it 

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

 an angle of 85, which would be the case in March and September, 

 fully one-half the rays which fell on the roof would be reflected ; 

 while, in the case of a ridge and furrow roof, if he shone on half the 

 roof, that is on one-half of each of the ridges, at any angle with a per- 

 pendicular 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 

 calculation, 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 which would enter through the ridge and furrow roof 

 would be 99,000, or very nearly double the number. If we compare 

 a roof in one plane with the framework of wood, with a similar one 

 with the framework of iron, and take the space rendered opaque by 

 the wood at twenty-one per cent., and by the iron at seven per cent., 

 then the greater number of rays admitted at all times by the iron roof 

 over the wooden one will be as three to one. 



Iron roofs have been objected to from their somewhat greater 

 original expense, from their supposed liability to break glass by con- 

 traction and expansion, and from the iron being liable to conduct 

 away heat in winter, and to become hot to such a degree as to be 

 injurious to the plants in summer. With regard to expense, that is, 

 we believe, now considered the chief objection ; but though it may be 

 heavier at first, yet it is amply compensated for by the great durability 

 of iron houses, when properly constructed, and when the iron is never 

 allowed to become rusty for want of paint. As a proof of the dura- 

 bility of iron houses, we may refer to the iron camellia-house, at 

 Messrs. Loddiges', erected in 1818, and the iron houses in the Horticul- 

 tural Society's garden, which were erected, we believe, in 1823. The 

 breakage of glass supposed to result from the contraction or expansion 

 of the metal was at one time considered a very weighty objection ; 

 but the severe winter of 1837-8 did not occasion so much broken 

 glass in iron as it did in wooden houses. A bar of malleable iron, 819 

 inches in length, at a temperature of 32, only increases in length one 

 inch, when heated to 212; but this difference of 180 of temperature 

 is more than plant-houses are liable to ; indeed 50 or 60 are as much 

 as is necessary to be taken into account. If we suppose the iron-work 

 is fitted at a period of the season when the temperature is 55, then 

 50 lower would be within 5 of 'zero, and 50 higher would be 105; 

 extremes which the iron roof of a hothouse will seldom exceed. Now, 

 according to the above data, a bar ten feet in length would extend or 

 contract, by the addition or reduction of 50 of heat, l-25th of an 

 inch as nearly as possible. An iron sash-bar, half an inch thick 

 between the two edges of the glass, would not expand in thickness, 

 from 50 of heat, much more than one six- thousandth part of an inch. 

 It may easily be conceived, therefore, that the lateral expansion of 

 sash-bars, which are in general not quite half an inch in thickness, by 

 any heat which they can receive on the roof of a hothouse, will never 



