-WARMING AND VENTILATION. 265 



becoming thoroughly heated, is conducted tlirough passages in which its hght- 



ntss causes it to ascend, and be delivered in any apartment of the house. 



„^ , , In the construction and arrangement of a furnace for heat- 



What two . ° 



points are of ing, the two ponits of special importance are, to secure a pcr- 



spccial import- ^^^^ combustion of the fuel, and tho best possible transmission 



ance in the con- ' '■ 



Btructioaoffur- of all the heat formed, into tho air that is to pass into the 



**'^^^''' rooms of the house. 



The first of these requisites is obtained by having a good draft and a fire- 

 ■box which is broad and shallow, so that tho coal shall form a thin stratum 

 »nd burn most perfectly. 



The second requisite is obtained by providing a great quantity of surface 

 "u the form of pipes, drums, or cylinders, through which the smoke and hot 

 gases must pass on tlieir way to the chimney, and to which their heat will be 

 imparted, to be in turn deUvered to the cold and pm-e air of the rooms of 

 tho house. 



592. The grrcat advantages of heating by steam are, that 

 What is the , , ° . , „ ,. ... 

 advantage of the heat can be communicated lor a great distance m anj"^ ai- 



heating by rection — upward, downward, or horizontally. As the tem- 

 steam? i i ' j 



perature of the heating surfaces, when low-pressure steam m 



used, never exceeds 212° F-, the air in contact with tliem is never contami- 

 nated by the burning of dust, or the abstraction of oxygen. 



Under favorable circumstances, one cubic foot of boiler will heat about 

 two thousand cubic feet of suitably inclosed space to a temperature of 70° to 

 80° F. 



593. We. apply the term fuel to any suh- 



Whatisfucl? tri J •' 



stance which serves as aliment or food for fire. 

 In ordinary language we mean by fuel the peculiar suh- 

 fitance of plants, or the products resulting from their de- 

 composition, designated under the various names of wood, 

 coal, &c. 



In recently cut wood, from ono fifth to one half of its weight 

 tion'' of'^^t'he is water ; after wood has been dried in the air for ten or 



weight of wood twelvemonths, it will even then contain fi-om 15 to 25 per 

 IB water y ' 



cent, of water. 



The amount of moisture in wood is greatest in the spring and summer, when 



the sap flows freely and the influence of vegetation is the greatest. Wood, 



therefore, is generally cut in the winter, because at that season there is but 



little sap in the tissues, and the wood is drier than at any other period. 



"Woods are designated as hard and soft. This distinction is 



Snated°°as^ grounded upon the facility with which they are worked, and 



hard and soft ? upon their power of producing heat. Hard woods, as tho 



oak, beech, walnut, elm, and alder, contain in the same bulk more .solid fiber, 



and their vessels are narrower and more closely packed than those of the 



softer kinds, such as pine, larch, chestnut, etc. 



12 



