Descriptive geometry . To develop the prism. Method. Determine the true size of aright section and proceed as in the case ofcylinder (Art. 125, page 88), or revolve theprism parallel to a coordinate plane and pro-ceed as for the cylinder (Art. 126, page 91). 129. The helical convolute. Fig. 166 illustrates a plane triangle tangentto a right-circular cylinder, the base of thetriangle being equal to the circumference ofthe base of the cylinder. If the triangularsurface be wrapped about the cylinder, thepoint c will come in contact with the cylinderat <?j, a at a^ and the hypotenuse ac will be
Descriptive geometry . To develop the prism. Method. Determine the true size of aright section and proceed as in the case ofcylinder (Art. 125, page 88), or revolve theprism parallel to a coordinate plane and pro-ceed as for the cylinder (Art. 126, page 91). 129. The helical convolute. Fig. 166 illustrates a plane triangle tangentto a right-circular cylinder, the base of thetriangle being equal to the circumference ofthe base of the cylinder. If the triangularsurface be wrapped about the cylinder, thepoint c will come in contact with the cylinderat <?j, a at a^ and the hypotenuse ac will be-come a helix having a^c^ for the pitch, and theangle acd will be the pitch angle. If the right line ac be revolved about the cylinderwhile remaining in contact with, and tangentto, the helix,* and making the constant angle6 with the plane of the base, it will generate aconvolute of two nappes. The lower nappe,which is generated by the variable portion ofthe line below the j^oii^t of tangency, willalone be Fig. 166. * For the theory and construction of the helix andinvolute curves see page 104 of Elements of MechanicalDrawinsr of this series. 94 DESCRIPTIVE GEOMETRY 130. To draw elements of the surface of thehelical convolute. Construction. Fig. 167. Let the di-ameter, aH^^ and the pitch, aV, of the re-
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