THE ANGLE OF ELEVATION. 



135 



calculated for the admission of the sun's 

 rays is a hemispherical figure;" and this 

 Mr Loudon at first considered as " the 

 ultimatum in regard to the principle 

 and perfection of form." The following 

 quotation bears on this subject : — " The 

 theory of the transmission of light through 

 transparent bodies is derived from a well- 

 known law in optics; and the influence of 

 the sun's rays on any surface, both as 

 respects light and heat, is directly as the 

 sine of the sun's altitude ; or, in other 

 words, directly as it is perpendicular to 

 that surface. If the surface is transparent, 

 the number of rays which pass through the 

 substance is governed by the same laws. 

 Thus, if one thousand rays fall perpendi- 

 cularly upon a surface of the best crown 

 glass, the whole will pass through except 

 about a fortieth part, which the impurities 

 of even the finest crystal, according to 

 Bouguer, will exclude ; but if these rays 

 fall at an incidental angle of 75°, two 

 hundred and ninety-nine rays, according 

 to the same author, will be reflected. 

 The incidental angle, it will be recollected, 

 is that contained between the plane of the 

 falling or impinging ray, and a perpendi- 

 cular to the surface on which it falls. 

 The benefit derived from the sun's in- 

 fluence on the roofs of hothouses depends, 

 as far as respects form of surface, entirely 

 on this principle. Boerhaave applied it 

 to houses for preserving plants through 

 the winter, and, of course, required that 

 the glass surface should be perpendicular 

 to the sun's rays at the shortest day, 

 when most heat and light were required. 

 Miller applied it to plant stoves, and pre- 

 fers two angles in the roof — one as the 

 upright glass, to meet the winter's sun 

 nearly at right angles, and the other as 

 the sloping glass, to meet it at an angle 

 of 45° for summer use, and the better 

 to admit the sun's rays in spring and 

 autumn. Williamson prefers an angle of 

 45° in all houses, as do most gardeners, 

 probably," Mr Loudon observes, "from 

 habit; but Knight prefers, in forcing- 

 houses at least, such a slope of roof as 

 shall be at right angles to the sun's rays, 

 at whatever season it is intended to ripen 

 the fruit. In one of the examples given, 

 ("Horticultural Transactions," vol. i. p. 

 99,) his object was to produce " a large 

 and highly-flavoured crop, rather than a 

 very early crop of grapes ; and he, accord- 



ingly, fixed upon such a slope of the roof 

 as that the sun's rays might be perpendi- 

 cular to it about the beginning of July, 

 the period about which he wished the 

 crop to ripen. The slope required to 

 effect this purpose, in latitude 52°, he 

 found to form an angle of 34° with the 

 plane of the horizon. In the application 

 of the same principle to the peach-house, 

 (" Horticultural Transactions," vol. i. p. 

 206,) in order to ripen the fruit about 

 midsummer, the roof was made to form 

 an angle with the horizon of 28°. Both 

 these houses, Knight informs us, pro- 

 duced abundant crops perfectly ripened." 

 — Encyclopaedia of Gardening, p. 583. 



Abercrombie says, " The diagonal side 

 of a glass case designed for a course of 

 forcing to begin 21st December, may be 

 55°; 22d January, 50°; 21st February, 

 46°; 21st March, 40°." Too much impor- 

 tance, he says, must not be attached to 

 the angle of inclination in the glass work. 



W. P. Ayres, in arranging the roof of his 

 cucumber-house so as to obtain the per- 

 pendicular solar rays in December in lati- 

 tude 53°, adopts an angle of 75° 28'; for 

 January, 71° 52'; for February, 62° 29'; 

 and for March, 51° 41'; — which latter 

 comes again into operation in September. 



The calculation of the inclination of 

 angles, for the better determining the 

 slope of hothouse roofs, is very differently 

 practised on the Continent and with us. 

 We calculate from the perpendicular line 

 of the quadrant, while they calculate 

 from the base or horizontal line. A very 

 excellent article on this subject will be 

 found in " Paxton's Magazine of Botany," 

 vol. ii. p. 257, from which the following 

 extract and diagrams are taken. Alluding 

 to the above modes of calculating, Sir 

 Joseph observes : " Both systems are 

 equally good when understood ; but an 

 understanding is requisite, because an 

 angle of 70°, which with us is well 

 known to be a very flat roof, is with the 

 French very steep ; and the same slope as 

 our 7 0°, on their system, is an angle of 

 20°. Likewise, an angle of 15°, which 

 in our calculation is very steep — only 

 fit for early peach-houses and other fruit- 

 houses, where the trees are trained to the 

 back wall — is, by the French calculation, 

 very flat, and unfit for early forcing." See 

 the angle 15, in the annexed scale, fig. 129. 

 " An example or two will probably 



