136 



HOTHOUSE-BUILDING. 



explain the subject better than words, viz., 17 feet higher at the back than 

 Supposing a house for early forcing was the front, as a b ; the same angle, 9 feet 

 required to be built 12 feet wide, and the wide, the back 12 feet 8 inches higher, 



Fig. 129. 



Fig. 130. 



12 feet 



roof to form an angle of 55°. Now, to 

 accomplish this, it would be necessary to 

 raise the back wall as much as 17 feet 

 higher than the front, as a b. Again, for 

 a house of the same dimensions, with an 

 angle of 45°, the back wall would require 

 to be 12 feet higher, as ac; with an 

 angle of 35°, only 8 feet 4 inches higher 

 than the front, a d ; with an angle of 20°, 

 only 4 feet 4 inches higher, a e ; with an 

 angle of 1 0°, which is the usual slope of 

 frames, only 2 feet 2 inches. 



" Of course, houses of less width require 

 less elevation at the back to give the 

 required angle. For instance, a house 

 6 feet, with a roof at an angle of 45°, will 

 only need the back wall to be 6 feet 

 higher than the front. Also, a frame 3 

 feet wide, having an angle of 10°, would 

 only require the back to be 7 inches 

 higher than the front. 



" It will always be useful to remember 

 that a perpendicular equal to the width of 

 the house always gives an angle of 45°. 



" The English gardeners calculate in- 

 clinations from the perpendicular, which, 

 in drawing plans for building, may be 

 explained by the annexed scale, (fig. 130,) 

 wherein the side V of the quadrant is 

 made the base, instead of a', which, of 

 course, reverses the whole order of calcu- 

 lation, making the first inclination 85° 

 instead of 5°. Supposing it is desired to 

 build a house 12 feet wide, with the 

 angle of the roof 35°, the elevation of the 

 back wall must be precisely the same as, 

 in the French scale, is required for 55° ; 



\ \ \ \ 



\ \ \ \ w i 

 \ \ \ \ \ 1 1 



\ \ \ \ \ \ i i 



12 feet 



the front, as a b ; if only 6 feet wide, 

 the back need only be 8 feet 6 inches 

 higher than the front, as ef; and so on 

 in proportion, measuring according to 

 the annexed scale. 



" It will also be seen that, by this scale, 

 80° is the common angle of a frame, and 

 exactly corresponds with 10° on the other 

 mode of calculation, and therefore, of 

 course, requires the same elevation — viz., 

 7 inches at the back more than the front. 



" To find the angle of the roof of any 

 hothouse, it is merely necessary to place 

 the side a' of the quadrant in a parallel 

 line against the side of the roof, for our 

 way of calculation, and the side^' for 

 that of the French, and the plum-line, 

 hanging perpendicularly, will immediately 

 give the required angle." 



These two scales deserve attentive 

 study, and are so simple, and at the 

 same time so complete, that all who are 

 interested in the subject should atten- 

 tively peruse them. 



The following very simple directions 

 forfinding the angle 

 Fig. 131. 0 f roofs are laid 



down by Mr Lou- 

 don. " The instru- 

 ment," he says, 

 " used by gardeners 

 for taking the angle 

 of roofs is a maho- 

 gany quadrant (fig. 

 131) of about 6 

 inches radius, with 

 a line and plummet, the quadrant being 



