GARDEN STRUCTURES. 



201 



inclined so as to face the south at an angle 

 equal to that of the latitude of the place, will 

 in the course of the whole year admit a greater 

 number of the rays of light to pass through 

 than would be the case at any other angle; but 

 according to the above table, we may deviate as 

 much as 20° higher or lower than that angle 

 without any material difference. In other words, 

 in latitude 54° the angle of roof may be as high 

 as 74° or as low as 34°, without the transmission 



of light being materially effected. Therefore, 

 between these limits we may choose any angle ; 

 according as the structure is intended for dwarf 

 or tall plants. 



On the whole, it does not appear desirable to 

 construct houses with roofs at a lower angle 

 with the horizon than 30° for the general crop, 

 nor higher than 40°, but for very early forcing 

 an angle of 45° may be allowed. A simple 

 mode of finding a standard angle for a spari- 



ng. 261.— Section of Lean-to House. 



roof is to raise the apex to one-third of the 

 width, so that in a span 15 feet wide the apex 

 (or ridge) would rise 5 feet, or in a lean-to roof 

 15 feet wide it would rise 10 feet from the 

 eaves to top of back wall. 



Plant Houses. — The outline of the roofs of 

 these is either straight or curved, and again the 

 glazed part may consist of one plane or uniform 

 slope, in which case the house is called a Lean- 

 to, or of two plane slopes in opposite directions, 

 when it is said to be Span-roofed. The span-roof 

 may either be equal, when both sides or slopes 

 of the roof are of equal pitch and length, or 

 unequal, when one side, usually that to the south, 

 presents a long slope, and the other side a short 

 one. 



The Lean-to Roof (fig. 261) is the form that 

 would most probably be suggested after the 

 shelter of walls had been taken advantage of; 

 and there is no form by which a certain amount 

 of light can be so readily admitted, and by 

 which an elevated temperature can be so steadily 



and so economically maintained, as by this. The 

 back wall generally faces the south, so that, 

 whilst the glass presents a surface in the best 

 position for receiving the rays of heat and light, 

 the north side, from the nature of the materials, 

 does not rapidly conduct heat from the interior, 

 more especially when its wall is built hollow, or 

 when it is protected to a considerable extent 

 from wet and cold by buildings erected at the 

 back. 



Span-roofed House (fig. 262). — The number of 

 superficial feet of glass required to cover a 

 certain area with a span-roof is exactly the 

 same as when the lean-to form is adopted, the 

 angle of elevation being the same in both cases, 

 but the extreme height of the span-roof is only 

 half that of the lean-to. This renders span- 

 roofed houses very advantageous for the growth 

 of plants that are dwarf, and cannot conve- 

 niently be placed in a lean-to so near the glass 

 as they ought to be. Another advantage is the 

 admission of light on both sides. When the 



