By Thomas Andrew Knight, Esq. 283 



sufficient to give heated air to a bed twenty feet long ; the 

 heated air entering at all times very rapidly, and consequently 

 always keeping all within the frame in motion. The larger 

 pipes might, I conceive, be with advantage made of cast iron. 



If the heat of the air be at any time excessive, it may be 

 lessened by opening the end of the tube at G, where it is 

 usually kept closed. The hot-bed, in which I have placed the 

 above described kind of tubes, is composed almost wholly of 

 leaves ; but the mass of these is great, and the temperature 

 in consequence high. I immersed a deep pot into the leaves, 

 and caused the heated air of the tube K, to ascend into it, 

 having previously shortened the tube, and fitted it accurately 

 to the aperture of the pot, placing a thermometer, with some 

 eggs of the common domestic fowl within it, with the view 

 of ascertaining whether these could be hatched by such means. 

 I have not yet seen the result ; but the temperature of the 

 ascending current of air, which arises into the pot, and 

 of course into the frame, appears never to have varied 

 during fifteen days more than three degrees, the lowest tem- 

 perature being 101°, and the highest 104° ; and it has, of 

 course, been nicely adapted to both the purposes for which it 

 was intended. 



I have formerly ascertained, that the power of a current 

 of heated air when made to enter a pit, or chamber of any 

 kind, was found greatly to exceed the calculation which I had 

 previously made ; and in the last winter, very contrary to my 

 expectations, a very feeble current of air, the temperature 

 of which was below 50° proved sufficient to preserv e gera- 

 niums, which were placed close to the glass, in the severest 

 frost, from receiving the slightest injury. 



The operation of a hot-bed, into which a pipe is introduced 



