328 FORCING GARDEN. 



but for pits they are eminently useful, from the small 

 space they occupy.; and when fired with coke, gas- 

 cinders, or anthracite, they give off very little smoke. 



It is unnecessary to describe all the numerous modi- 

 fications of this apparatus ; but it may be proper to 

 direct the attention of the reader to the close boiler 

 represented in Fig. 32, in which is shown how the 

 circulation may be conducted over a door or other ob- 

 stacle. In this case the upper pipe must not ascend 

 and descend twice : air-tubes ought also to be placed in 

 the boiler, and on the highest part of the pipes ; and 

 the whole must be made considerably stronger than on 

 common occasions. The annexed figure :will give an 

 idea of an isometrical elevation of a hot-water appara- 



Fis. 33. 



tus for a vinei-y thirty feet long by eleven wide. A is 

 the boiler, as in the figure on p. 324 ; B the upper or 

 delivering pipe ; C the principal part of the upper pipe, 

 of a flat form, presenting a greater radiating surface, 

 in proportion to the quanity of heat; D the descending 

 limb ; E the returning pipe, of a cylindrical form. 



Mr. Fowler has employed the siphon as a part of the 

 'hot-water apparatus; and in his tract on the Thermo- 



