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 a:5 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-j3ues. I have since pulled 

 them from oif 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 impioper to open the sashes 

 for the purpose. Ventilators are in that case use- 

 ful. They may be contrived in ditferent 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- wail ; and three in an 

 ordinary-sized house would be enough. 



I lately made four ventilators, in a house tliat 

 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 tlie manner of a 

 common chimney or fire-place, eighteen inches 

 wide, and two feet high, from whicli 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 



