2061. 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 reflection 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 appears by Bouguer's Table of Rays reflected from Glass. Of 1000 incidental rays when the angle of incidence is 87� 30' 584 are reflected. 75� 299 are reflected. 40� 34 are reflected. 85 543 70 222 30 27 82 30 474 65 157 20 25 80 412 60 112 10 25 77 30 356 50 57 1 25' (Hort. Trans., vol. i. p. 164.) 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 Wilkinson suggests, decidedly the best slope for general purposes.
