2060. The benefit derived from the sun's influence 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 (Dict. 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 45ﾦ for summer use, and 'the better to admit the sun's rays in spring and autumn.' Wilkinson (Hort. Trans,, vol. i. p. 161.) prefers this angle (45ﾦ) 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 flavoured 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 perpendicular 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 52ﾦ, he found to form an angle of 34ﾦ 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 midsummer, the roof was made to form an angle with the horizon of 28ﾦ. Both these houses, Knight assures us, produced abundant crops perfectly ripened.
