284 THE FORCING GARDEN. 



I had a suspicion of these truths ; and in order 

 to put them to the test, had tubes of various shapes, 

 sizes, and lengths made ; and which I could lay on 

 the fire-flue, on the ground, on a shelf, or, in short, 

 any where I liked. They were all fitted to the air- 

 chamber, formed round the furnace, which had a 

 turning valve in front, three inches diameter. That 

 with which most of the experiments were made, 

 was a square tube of three inches void, formed of 

 milled clay, and baked in the same way as garden 

 pots are. It was in lengths of three feet each, and 

 jointed with cement, quite close, in the same way 

 as a clay water-pipe. 



I tried its effect in conducting the heated air 

 from the furnace at various lengths, and at various 

 heights ; from two yards in length to forty feet ; 

 and from one foot above the level of the furnace, to 

 ten. The result of the whole amounted to this, 

 that unless when laid on the top of the fire-flue, air, 

 perceptibly hot, could not be drawn from the fur- 

 nace to a distance of more than twelve feet ; even 

 although the furnace were made ever so hot. 



When the tube was shortened to two lengths 

 (six feet,) the air issuing from it raised the mercury 

 in the thermometer to 120; but when another 

 length was put on, though the fire was kept equally 

 brisk, it fell to 96 ; and by adding a fourth length 

 (the tube being now twelve feet long), the mercu- 

 ry was then not affected by the air of the tube, but 

 fell to the air of the house, 65. 



The above experiment was made with the tube 



