CONSTRUCTION AND HEATING OF GLASS HOUSES. 61 



lower eud is of the most vital importance, its observance or 

 neglect making a difference of many years in the life of a glass 

 house, and yet I never remember having seen it practised by any 

 other builder than myself. 



Next in order we come to the purlin. This may be of wood, 

 about two by three inches if desired, but as the cost of an iron 

 purlin drilled to accommodate the correct spacing of the glazing 

 bars for the proper reception of the glass, is so very little in excess 

 of that of the wood purlin, and considering that the iron will cast 

 much less shade than the wood, I would advise giving it the 

 preference. If an iron purlin is decided upon, and you Avish to 

 order from a firm of experienced greenhouse builders, send them a 

 small piece of the bar you intend using, state the size of the glass, 

 and trust them to drill the purlin correctly. They will also arrange 

 for suitable splicing of the iron, and for caps to receive your 

 iron pipe columns, which are necessary to support the purlins. If, 

 however, you desire to do the drilling yourself, or have yonr local 

 mechanics do it for you, to the width of bar in sight between the 

 rabbets, usually about half an inch, add the width of the glass, 

 say sixteen inches plus one and one-sixteenth inches, for play ; 

 this will give you sixteen and nine-sixteenths inches between 

 centres of bars, and this will be spacing of holes necessary for 

 securing bars to purlins. The size of the holes should be three- 

 sixteenths of an inch, which will accommodate the No. 8 round- 

 headed screws. The section of iron best suited for the purlin is 

 two by one and one-fourth inches by one-fourth angle ; if this 

 cannot be easily secured, one and three-fourths by one and three- 

 fourths inches by one-fourth angle ma}'^ be substituted. The size 

 of the columns for the support of these purlins, depends upon the 

 extent of roof they have to support. For the southern slope of 

 an eighteen feet six inches wide, three-quarter span house, one 

 and oue-fourth-inch gas pipe will answer, or, for each slope of an 

 even span house of the same widths, one-inch pipe is ample, placed 

 about eight feet apart. 



We next come to the header which makes the connection of 

 the roof ventilating sash and the roof. In houses such as we are 

 now considering, I prefer to run all bars up to the roof and fit 

 the header over the bars. 



The ridge of such houses as these plays a very important part, 

 as on it devolves the duty of giving and maintaining a true roof 



