94 GARDENING FOR PLEASURE. 



on edge, the top being covered by a brick laid flat ; this 

 is the smallest size of flue. Larger grate surface will re- 

 quire correspondingly greater hight and breadth. Flues 

 are now commonly made by using cement or vitrified 

 drain pipe, to connect with the brick flue, at from 25 to 

 40 feet from the furnace, the pipe not being safe to use 

 near the furnace, as the greater heat would be likely to 

 crack it. A flue, to get the full benefit of the fuel, should 

 be so arranged that it goes all around the greenhouse, the 

 base of the chimney being the top of the furnace. The 

 advantage of this plan, (fig. 42), which has only recent- 

 ly had general publicity, is that the excessive heat given 

 out from the top of the furnace, drives back the cold air 

 that would otherwise pass down the chimney ; not only 

 too FEET 



Xig. A. PLAN OF GREENHOUSE HEATED BY DOUBLE FLUES. 



drives it back, but passing rapidly out, "draws" to it the 

 heated air that has to pass through the length of the 

 horizontal flue, causing it to circulate so rapidly that all 

 parts of the flue become nearly equally heated. In the 

 case represented in fig. 42, the greenhouse so heated 

 is 20 feet wide by 100 feet long, having a glass sur- 

 face of over 2,500 square feet, a size utterly impossi- 

 ble to heat with a flue unless so constructed that the base 

 of the chimney stands on the top of the furnace. It will 

 be seen by the plan that there are two flues running from 

 one furnace, and entering into one chimney. I only il- 

 lustrate this to show the power given by this method. 

 It would do quite as well, if the house was half or 

 quarter the size, to have it done by one flue instead of 

 the two. I would here say emphatically that no matter 



