76. How do you apply the principle of the parallelogram of forces in determining the strain on the various members of a structure? Illustrate graphically. 77. What should be the thickness at the top and base of a retaining wall 15 feet high, built to retain ordinary earth? Show your method of obtaining the required dimensions, also a sketch of the wall, showing how it should be founded. 78. A reservoir is to be built, depth of water 10 feet. If the walls are built of masonry, find the thickness of the same, and state how they should be built. Show your work. 79. What is an arch, of how many forms, and of what may it be constructed? 80. Can you state how you would find the thickness of an arch of stone, span and rise being given? 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 good brick from a poor one. 93. In how many ways is brickwork "bonded" to make good work in heavy walls? 94. What is hydraulic cement, and how many kinds do you know of? 95. Which do you consider the better quality, Rosendale or Portland, and why? 96. What is mortar composed of, and how mixed? 97. What kind of sand should be used, and how do you test its quality? 98. What is the meaning of the term "setting" as applied to cement? 99. How would you test cement? 100. What is concrete, of what composed, and in what proportion should its ingredients be mixed? 101. Name the common kinds of wood used in building. 102. What kind of timber resists decay longest under ground? 103. How may timber be preserved from decay? 104. What do you understand by limit of elasticity as applied to a beam under strain or pressure? What is meant by the neutral axis of a beam? 105. What is the tensile strength of a good quality of wrought iron per square inch? 106. For what parts of a structure may cast and wrought iron be used in reference to tension and compression? 107. Make a sketch of the form of cast-iron beam best adapted to resist a transverse strain. * * * * * CELEBRATION OF THE FIVE HUNDREDTH ANNIVERSARY OF THE UNIVERSITY OF HEIDELBERG, AUGUST, 1886.