266 WELLS'S NATURAL PHILOSOPHY. 



What Is the 594. The weight of wood varies greatly ; 

 weTJiuofwood? from forty-four hundred pounds in a cord of 

 dry hickory, to twenty-six hundred in a cord 

 of dry, soft maple. 



What is the ^^5. For fuel, the most valuable of the com- 

 vaiurof"wood ^^^ kinds of wood are the varieties of hickory; 

 for fuel? after that, in order, the oak, the apple-tree, 



the white-ash, the dog-wood, and the heech. The wooda 

 that give out the least heat in burning are the white-pine, 

 the white-birch, and the poplar. 



, ., -^ , , 596. The remark is sometimes made that " it is economy to 



Is it profitable •' 



to bum green bum green wood, because it is more durable, and therefore 

 ■wood? jjj ^y^Q gQ(j more cheap." This idea is erroneous. The con- 



sumption of green wood is less rapid than dry, but to produce a given amount 

 of heat, a far greater amount of fuel must be consumed. 



The evaporation of liquids, or their conversion into steam, consumes or ren- 

 ders latent a great amount of caloric. When green wood or wet coal is added 

 to the fire, it abstracts from it by degrees a sufficient amount of heat to con- 

 vert its own sap or moisture into steam before it is capable of being burned. 

 As long as any considerable part of this fluid remains unevaporated, the 

 combustion goes on slowly, the fire is dull, and the heat feeble. 



,^^ , 597. Coal and hard wood are not readily ignited by th« 



Why are coal •' ° •' 



and hard woods blaze of a match, because on account of their density they are 

 difficult to ig- rendered comparatively good conductors, and thus carry off 

 match? the heat of the kindling substance, so as to extinguish it, 



before they themselves become raised to the temperature 

 necessary for combustion. 



Light fuel, on the contrary, being a slow conductor of heat, kindles easily, 

 and, from the admixture of atmospheric air in its pores and crevices, burns 

 out rapidly, producing a comparatively temporary, though often strong heat. 



CHAPTER XIII. 



METEOROLOGY. 



What is Me- ^9^- Meteorology is that department of 

 teoroiogy? physical science which treats of the atmos- 

 phere and its phenomena, particularly in its relation to 

 heat and moisture. 



699. By climate, we mean the condition of a place in 



