BUILDING MATERIALS 



269 



part. Wood is still one of the most used building ma- 

 terials in this country. Frame houses are much more 

 numerous than those of brick, stone, and concrete. The 

 quality of the wood depends on the kind of tree which 

 produces it, and the beautiful grain that is visible in some 

 woods is nothing more or less than the so-called annual 

 rings which appear as a result of growth in certain kinds of 

 trees. The material in the board or beam or in the mold- 

 ing or mantle of a room is composed of millions of dead 

 cells, the walls of which only remain. These walls are 

 hollow, living matter has gone from them, and when 

 wood is, finished, these holes are filled up with paint, oil, 

 wax, varnish, and whatever substance is used to give wood 

 its finished appearance. Although wood is necessary for 

 some parts of the house, it is not the safest material for 

 construction because of its liability to burn. Wooden 

 shingles should be treated with a solution to make them 

 wear longer, and to protect 

 us from fire. 



Experiment. To show the rela- 

 tion of the markings in trim 

 to the cutting of wood. 



Materials : Diagrams, school 



furniture. Hough's sections 



of woods. 

 Method : Examine the sections 



shown you. Compare with 



figure showing cross section 



radial and tangential cutting 



of timber. 



Note : Most lumber is cut tangentially . Hence the yearly rings take 



a more or less irregular course. The grain of wood is caused by the fibers 



not taking straight lines in their course in the tree trunk. In many cases 



the fibers of the wood take a spiral course up the trunk, or they may wave 



outward to form little projections. Boards cut from trees with irregular 



Diagrams of sections of timber, a, Cross 

 section, b, Radial, c, Tangentials. 



