ARTIFICIAL HEAT AND APPARATUS 45 



hermetically sealed glass bottles, the generated heat is 

 constantly passing off through the crevices, more particu- 

 larly those in the upper part of the roof, and the moisture 

 along with it, The main difference, then, between the 

 two is, that one only raises the temperature, while the 

 other does the same ; and, in addition, imparts a certain 

 amount of sulphuretted hydrogen and other injurious gases 

 that escape through the joints and the bricks, in quantity 

 according to the good or bad workmanship, or the porosity 

 of the material. " Smoke has thin shoulders," is an old 

 adage among gardeners, and we often find it verified in 

 the use of flues. Notwithstanding these disadvantages, 

 good grapes may be forced by flue heat, and in some cases 

 this is the only means at command; when so, it is advisa- 

 ble to make the best arrangement we can. If a steady 

 and continuous heat be required, there ought to be enough 

 room in the furnace to hold a large bulk of fuel, and if it 

 is situated somewhat below the level of the flue, the heat 

 will flow more evenly, and disseminate itself more equally, 

 as it travels along with the draft, by which a greater por- 

 tion will be absorbed by the bricks, and given off into the 

 house. A small furnace, with a quick draft, is never eco- 

 nomical, as a considerable portion of heat passes off by the 

 chimney and is lost. A narrow and small flue is also ob- 

 jectionable, as the heat acts directly upon the mason-work, 

 and the bricks become so hot as to abstract the oxygen of 

 the air, rendering it so devoid of moisture as not to be fit 

 for the leaves to respire in. To a certain extent, this evil 

 may be counteracted, by keeping shallow vessels filled 

 with water upon the flue, and damping the floor of the 

 house ; but most of the moisture produced in this way is 

 in a vaporous state, and not in the exact solution required 

 by the absorbing stomata of plants. The analogy be- 



