SCIENCE OF COMMON THINGS. 



43 



Tenons. Scarfing. Tongueing. Arch. 



them, which otherwise would be liable to injure the 

 building by its weight. 



253 What is a mortise ? 



The opening or hole cut in one piece of wood to 

 admit the projecting extremity of another piece. 



254 What is a tenon? 



The end of a piece of timber which is reduced in di- 

 mensions so as to be fitted into a mortise for fastening 

 two timbers together. 



255 What is scarfing and interlocking t 



It is that method of insertion in which the ends of 

 pieces overlay each other, and are indented together, so 

 as to resist longitudinal strain by extension, as in tie 

 bearers and the ends of hoops. (See fig. 11.) 



256 What is tongueing and rabbeting f 



It is that method of insertion in which the edges of 

 boards are wholly or partially received by channels in, 

 each other. 



257 What is an arch t 



It is a part of a structure or building suspended over 

 a hollow, and concave towards the area of the hollow. 



253 Is it known at what time the arch was invented ? 



It is not ; it does not appear to have been known to 

 the ancients. 



259 Why is an arch capable of resisting a greater amount of pressure 

 than a horizontal or rectangular structure constructed of* the same ma- 

 terials f 



Because the arrangement of the materials composing 



