GENERAL PRINCIPLES. 



115 



the obstruction to the rays of light enter- present themselves more perpendicularly 

 ing the house at an early and a late to the glass at those times when they 

 hour of the day." This led him to the are the least powerful ; whereas at mid- 

 adoption of the ridge-and-furrow prin- day, when they are most powerful, they 

 ciple for glass roofs, which places the present themselves more obliquely to it. 

 glass in such a position that the rays of The following diagram will show the 

 light . in the mornings and evenings construction of such roofs. Fig. 114 is 

 enter the house without obstruction, and the elevation of a lean-to house of this 



Fig. 114. 



description. The whole roof is of neces- 

 sity a fixture, excepting the angular 

 lights under each ridge, which may be 

 made movable by being hinged at their 

 bottom sides, opening outwards, and 

 being kept at any angle of depression 

 by a sliding rod of iron attached to 

 their apex, and made to slide up and 

 down in a dove-tailed groove of iron, 

 fastened to the under side of the crown 

 of the ridge. This rod should have a 

 spring catch, acted upon from within, 

 which would open out the angular sashes 

 to any extent, and keep them so till 

 again acted upon in a contrary direction. 

 Ventilators of the common form are 

 placed in the back wall, opposite to each 

 ridge, and are opened and shut upon 

 the ordinary principle. This ventilation 

 is to be used in the warmest weather, 

 or when the greatest quantity of air is 

 wished to be admitted. In ordinary 

 cases, the ventilators shown in the front 

 parapet wall are used, and the corres- 

 ponding ones in the top of the back walls 

 — only the external openings of which 

 can be shown in the elevation ; and for 

 ordinary purposes these will be found 

 sufficient. Front ventilation may also 

 be easily obtained by making the front 

 upright sashes pass each other, running 

 on an iron rail fixed to the top of the 

 stone parapet, or by their being made 

 to open as shown in the elevation. 



Fig. 115 is the end elevation of the 

 same house, showing ventilation which 

 ought to exist at both ends, if the house 



Fig. 115. 



stands insulated ; also the ventilators in 

 the parapet wall, and the mode by which 

 the front sashes are thrown out. 



Fig. 116. 



Fig. 116 shows the ridge rafter, one- 

 fourth the full size ; and 



