81. Define the intrados and extrados of an arch.

82. Where should the line of resistance to pressure be found in an
arch in order to retain its stability?

83. Can you find the thickness of the abutments, the rise and span of
the arch being given?

84. In a semicircular arch, where is the horizontal thrust greatest
and where least?

85. Name the common kinds of stone used in building.

86. Define the terms "quarry-faced," "rough-pointed," "fine-axed,"
"bush-hammered," as applied to the dressing of stone.

87. Describe "rubble" masonry, "ashlar" masonry, and "broken ashlar"
masonry.

88. What are "headers" and "stretchers"?

89. What should be the proportion of "headers" to "stretchers"?

90. How would you prepare the foundation for a heavy wall, and how
deep should it be excavated?

91. How are walls founded on soft or yielding materials?

92. Describe a good quality of bricks, and state how you would know a