202 



THE GARDENER'S ASSISTANT. 



rays of light fall chiefly on one side of a plant, 

 it grows most to that side ; but in a span-roofed 

 house, light being admitted on both sides, 

 equality of growth is more easily maintained. 

 Much more fire-heat is, however, required for 

 the span-roof than for the lean-to, especially if 

 the house runs east and west, as in that case 

 half the surface of the glass is exposed to the 

 north. Accordingly the span-roofed form is 

 employed to most advantage in such structures 

 as conservatories, orchard-houses, and pits. 



Unequal Span-roofed Houses. — As already men- 

 tioned, these generally have the longest slope 

 facing the south, consequently they are more 

 economical as regards fuel than the equal span, 

 because there is less surface exposed to the 

 north. Their great advantage consists in the 

 extreme height of the roof being considerably 

 decreased by the high upper angle being cut 

 off, as in fig. 263, and thus the heated air 

 cannot accumulate so much at that part of the 

 house as if the roof had extended till it joined 



Fig. 262.— Section of Span-roofetl House. 



the back wall. They are more costly in con- 

 struction than the lean-to, and for general 

 purposes the latter form of house serves equally 

 well. 



Bidge-and-furrow Roofs. — In covering a certain 

 area with glass in one plane, as in the lean-to 

 houses, or even those with a double span, the 

 height of the roof must be considerable, in order 

 to throw off the wet ; but by adopting the ridge- 

 and-furrow system this object can be effected 

 with but little elevation of roof. A conservatory, 

 for instance, may be constructed with glazed 

 sides all of equal height, say 10 feet, and by 

 covering it in on the ridge-and-furrow system, 

 it is possible to do so without raising the top of 

 the ridges more than 18 inches higher than the 

 sides. This appears to be the only real advantage 

 afforded by this mode of construction. It may be 

 observed, that the quantity of glass required to 

 cover a certain area of base is the same, whether 

 this is effected by one span, by a series of spans 



like the ridge-and-furrow, or by a single slope 

 like a lean-to, provided the glass in all these 

 cases is placed at the same slope. With regard 

 to light, it will be readily admitted that through 

 every square foot of glass placed at the same 

 angle, the number of transmitted rays will be 

 the same. Now there is this difference between 

 a ridge-and-furrow roof and one with a single 

 slope : there is, as above stated, the same surface 

 of glass in both if placed at the same angle, but 

 when the sun's rays are at a right angle to one 

 side of the ridge none of them can be so to the 

 opposite one ; whereas on a roof of a single slope, 

 the whole of the surface is exposed to the sun's 

 rays, so that twice the quantity of these will be 

 transmitted into the interior of the house. In 

 consequence of this, it can be affirmed that a 

 ridge-and-furrow roof will be heated as much by 

 the sun's rays as a span-roofed one, but much 

 less so than a roof presenting one uniform slope. 

 The latter is best for the ripening of fruits, whilst 



