HOT-HOUS^ ROOFS. 341 



of convenience, tliey should be made considerably 

 shorter than the others. They are furnished with cords, 

 pulleys, rollers, and weights, though the last, with no 

 very prudent regard to economy, are sometimes omitted. 

 Formerly, all hot-houses were constructed with upright 

 sashes in front. One of the most eminent garden ar- 

 chitects of the present day (Mr. Atkinson) has discon- 

 tinued the practice ;_ and,' except in ornamental struc- 

 tures, it is hard to say why it should not t)e laid aside 

 altogether : for while upright sashes certainly tend . to 

 weaken the fabric, and increase its expense, their utility 

 is at least problematical. 



Glass is the transparent material universally employ^ 

 ed, for it is at once a ready transmitter of the rays of 

 the sun, and a bad conductor of caloric, or it admits 

 light, and retains the heat generated by flues. That 

 some tint of blue or green would lessen the scorching 

 effects of the rays seems generally admitted; but the 

 precise tint has not yet been satisfactorily established. 

 Formerly the panes of glass employed were of large 

 size, but small panes are -found to be more economical, 

 being less liable to break, and more easily replaced. It 

 is believed that a pane seven inches- in breadth by six 

 in -length is the cheapest form in which good glass can 

 be obtained. In glazing, it is important to keep the 

 overlaps of the panes of small dimensions, perhaps from 

 oncrfourth to one-eighth lof an''inch in breadth. This 

 diminishes the breakage which arises from the expansion 

 attending; the freezing!; of water detained between the 

 laps by capilldry attraction. As a further preventive, 

 the interstices are sometimes filled with putty, and oc- 

 casionally with laps of lead or copper. This effects a 

 considerable saving of ^glass and of heat, but imposes on 

 the gardener the duty of increased attention in prevent- 



