Book III. PRINCIPLES OF DESIGN IN HOT-HOUSES. 313 



crystal, according to Bouguer, will exclude ; but if these rays fall at an incidental angle 

 of 75°, 299 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 perpendicular to the surface on which it falls. 



1597. The benefit derived from the sun's influence on the roofs of hot-houses 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 [Diet. art. Sun,) applied it to plant-stoves, and prefers 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 him at an angle of 45° for summer 

 use, and "the better to admit the sun's rays in spring and autumn." Williamson 

 (Hort. Trans, vol. i. p. 161.) prefers this angle (45°) in all houses, as do most gardeners, 

 probably 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 (Hort. Trans, vol. i. p. 99.), his object was to 

 produce a large and highly flavored crop, rather than a very early crop of grapes ; and 

 he accordingly fixed upon such a slope of roof as that the sun's rays might be perpen- 

 dicular 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 (Hort. Trans, vol. i. p. 206.) in order to ripen the fruit about mid- 

 summer, the roof was made to form an angle with the horizon of 28°. Both these 

 houses, Knight assures us, produced abundant crops perfectly ripened. 



1598. As data to determine the angles of glass roofs, the following are laid down by 

 Wilkinson. The angle contained between the back wall of the forcing-house, and 

 the inclined plane of the glass roof, always equals the sun's altitude, when his rays fall 

 perpendicularly on that plane, provided that the inclination of the plane to the horizon be 

 at an angle not less than 28° 2', nor greater than 75°. Within the above limits, the 

 sun's rays are perpendicular twice in the year, once in going to, and once in returning 

 from, the tropic. Hence then, having determined in what season we wish to have the 

 most powerful effects from the sun, we may construct our houses accordingly by the 

 following rule. Make the angle contained between the back wall of the house and its 

 roof, equal to the complement of latitude of the place, less or more the sun's declination 

 for that day on which we wish his rays to fall perpendicularly. From the vernal to the 

 autumnal equinox, the declination is to be added, and the contrary. Thus, to apply 

 these principles to the slope of roof recommended by Knight, for ripening grapes in 

 July ; say at London we have 



Latitude of London 51° 29' 



Sun's declination on the 21st July - - 17° 31' 



33° 58' or 34" nearly. 



Wilkinson adds that " as we want the genial warmth of the sun most in spring, 

 therefore, for general purposes, that construction would perhaps be best which gives us 

 the greatest quantity of perpendicular rays then. If the inclination were 45°, the sun's 

 rays would be perpendicular about April 6th and September 4th. And as the rays 

 would vary very little from the perpendicular for several days before and after the 

 6th of April and September 4th, the loss of rays arising from reflexion, would, as appears 

 from the annexed table, be nearly a minimum. Even at the winter solstice, the loss by 

 the obliquity of the angle of incidence would be only two in 1000 more than when the 

 rays fall perpendicularly, as appeal's by Bouguer's Table of Rays reflected from Glass. 



Of 1000 incidental rays when the angle of incidence is 



Hort. Trans, vol. ii. p. 237. 



When, in addition to this, it is considered, that the slope of 45° is the least that will 

 effectually drain the water from the intervals between the lapping over of the panes of 

 glass, that angle appears to us, as Williamson suggests, decidedly the best slope for 

 general purposes. 



1599. Air is supplied by the portion of the atmosphere enclosed by the tegu- 

 ment. This air may be raised in temperature, charged with vapors, or renewed, at the 

 will of the operator. It might also be put in motion by art, for the sake of obtaining 

 strength of stem in ramose or tree-like plants ; but the motion communicated to plants, by 

 opening the cover, and exposing them to the direct influence of the air in fine weather, is 

 deemed sufficient, either for this purpose, or giving flavor to fruits when advancing to 

 maturity. A very fit machine for putting air in motion, or for extracting air, was 

 invented by B. Deacon (Patent- office, 1812, and Remarks on Hot-houses, part 2.) It 



