73. Suppose a beam supported at both ends, and take w = weight, l =
length of beam, b = breadth, d = depth, s = breaking weight. Express
algebraically the value of s in terms of the other quantities.

74. Sectional area being 36 square inches, which would be the stronger
section, 6 by 6 or 4 by 9?

75. Make a design for a pair of rafters, connected by a tie-beam, for
a roof 30 feet span, showing the dimensions of the several parts and
the manner of connecting them. State in detail your method of
obtaining the several dimensions.

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?