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 

 witii which most of the experiments w^ere made, was 

 a square tube of three inches void, formed of mill- 

 ed clay, and baked in the same way as garden pots 

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

 ed 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 abo\ e 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-fiue, 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 1'20° ; but when another 

 length was put on, though the fire was kept equally 

 brisk, it fell to 9^>" ; and by adding a fourth length 

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

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

 fell to l!ie air of the house, 65". 



The above experiment was made with tlie tube 



