CONSTRUCTION OF HOT-HOUSES. 285 



at six feet above the level of the top of the furnace. 

 It was tried at two feet above it, with hardly any 

 variation ; but when laid on the ground, on the 

 same level with the top of the furnace, the heat did 

 not travel to the length of nine feet, so as to affect 

 the spirits in the thermometer, the air of the house 

 being then at 62. Thus, I think I have ascertain- 

 ed the uselessness of air-flues. I have since pulled 

 them from off the fire-flue, but have never erected 

 one upon it. 



Of Ventilators. 



The hot-house may require to be ventilated at 

 times, when it may be improper to open the sashes 

 for the purpose. Ventilators are in that case use- 

 ful. They may be contrived in different forms, 

 and may be placed in different situations. If the 

 hot-house have a shed behind it, they might be 

 made to open, in the manner of a common window, 

 near to the top of the back-wall ; and three in an 

 ordinary-sized house would be enough. 



I lately made four ventilators, in a house that 

 had no shed behind it, in this manner : When 

 the wall was raised to within a yard of its full 

 height, apertures were formed in the manner of a 

 common chimney or fire-place, eighteen inches 

 wide, and two feet high, from which a small vent 

 was carried through the coping. On the top was 

 fixed a horizontal tube, three inches square, and two 

 feet long, with a centre pipe fixed into the vent. 

 The aperture or chimney was filled in front, with 



