BOOK III. 



FORMS OF HOT-HOUSE ROOFS. 



317 



the time between his rising and mid-day, and perpendicular to the other half, at half 

 the period of time between mid-day and sunset. Another advantage of ridge and furrow 

 roofs is, that the laps between the panes, unless very broad or puttied, are always kept 

 free from accumulations of dust. This takes place in consequence of their angle of in- 

 clination, which being about 45, the gravity of the column of water between the laps 

 is found to counterbalance the attraction of cohesion, and slides in the lap from the 

 crown to the bottom of the furrow. 



1609. The general form 259 

 and appearance of a ridge and 



furrow house (jig. 259.) is _ ~ : . _-. -=^=*-=L 



not materially different from 



that of others. Where the 



curved end is adopted, it will 



not be necessary to deviate 



from the common mode of 



glazing in these parts of the 



roof, unless with a view to resist a weight of snow. 



the roof, therefore, is ridged 



(Jig. 260. a, a), the ends will 



present a smooth surface 



(Jig. 260. 6, 6). 



1610. The polyprosopic 

 hot-house (Jig. 261. ) re- 

 sembles a curvilinear house, 

 but differs in having the 

 surface thrown into a num- 

 ber of faces, the chief advan- 

 tages of which are, 1. That 

 by hinging all the different 



faces at their upper angles, and by having rods connecting the lower outside corners of 

 the faces terminating in chains which go over pulleys in the top or above the back wall, 

 the whole roof, including the ends, may be opened or raised sympathetically, like Vene- 

 tian blinds (jig. 261. a.), either so as each sash or face may be placed in the plane of the 

 angle of the sun's rays at the time, or to the perpendicular, to admit a shower of rain. 



While the parallelogram part of 

 260 



In consequence of this arrangement, the plants in a polyprosopic house may, at 

 any time, and in a few minutes, be placed in effect, or as far as respects light, air, wind, 

 rain, dew, &c. in the open air ; and being so placed, may, whenever desired, be as 

 speedily restored again to their proper climate. The arrangement by which this is effected, 

 and which is perfectly simple, is applicable to every form of hot-house, whether of glass on 

 all sides, on two, or on three sides ; or whether the roof is formed of curved or straight lines. 

 We consider it, indeed, to be the ne plus ultra of improvement, as far as air and light are 

 concerned. One objection to all curvilinear forms in this respect is, that the roof, unless 

 a considerable expense be incurred, must be fixed, and air admitted by horizontal wooden 

 or glazed shutters in the parapets, or between the props, and allowed to escape by sky- 

 lights or shutters at the top of the back wall ; but here the air is equally admitted in every 

 part of the house, in the most natural manner, without the creation of currents or eddies, 

 and without excluding any more sun than will be obstructed by the thickness or edge of 

 the faces or sashes. In like manner, a great objection to straight-lined roofs with sliding 

 sashes is, that air can only be partially admitted, and that while this is being done, one 

 glazed frame being slid over the other in all those parts where there is a double portion of 

 glass, a double portion of light must be excluded ; and as opticians are aware, the light 

 so transmitted will be doubly decomposed by passing through two surfaces of glass. 



1611. This roof, with respect to the sun's rays, may be considered as exactly equivalent 

 to a curvilinear figure whose curve lines shall touch all the angles of the faces, so that the 

 sun in general would be nearly perpendicular to some one face every hour in the day, 

 and every day in the year. A specimen of glass roof, constructed on this principle, 

 formed a part of the erection at Bayswater , (1602.), already referred to, but which 

 owing to local alterations it became necessary to remove in 1823. 



