The essentials of descriptive geometry . he plane T = —2; —13s; +120. Assume a line in this plane ibehind and parallel to V. 80. The plane S = o; +60; —60. Assume a line in the plane 2 belowH and parallel to H. 81. The plane s = —2; +120; —135. Assume a line in this planeparallel to V and i behind V. 82. Locate the planes »; -3; -400; -3; 0000 ; 00 ; +300 ; 00 ; —200; +2; -I. 83. The plane S = 00 ; +3; —4. Find the coordinates of a line 3long in this plane 2 from the G. L. 84. The plane T = 00 ; +2; —3. Draw a hole 1 square in this plane,and show three views of it. CHAPTER VI REVOLUTION OF POI
The essentials of descriptive geometry . he plane T = —2; —13s; +120. Assume a line in this plane ibehind and parallel to V. 80. The plane S = o; +60; —60. Assume a line in the plane 2 belowH and parallel to H. 81. The plane s = —2; +120; —135. Assume a line in this planeparallel to V and i behind V. 82. Locate the planes »; -3; -400; -3; 0000 ; 00 ; +300 ; 00 ; —200; +2; -I. 83. The plane S = 00 ; +3; —4. Find the coordinates of a line 3long in this plane 2 from the G. L. 84. The plane T = 00 ; +2; —3. Draw a hole 1 square in this plane,and show three views of it. CHAPTER VI REVOLUTION OF POINTS 30. One of the most important operations in descriptivegeometry and the means by which a great many problems aresolved is the revolution of a point, or a Hne, from one position toanother. The actual laws governing such a movement are wellknown, but to represent the change of position correctly, and tovisualize, or to get a mental picture of what is taking place dur-ing the change of position, requires careful Fig. 35. 31. When a point revolves about a Hne two things are ap-parent at once: 1. That the point describes a circle in space. 2. That the plane of this circle is perpendicular to the in other words, the revolution of any point about an axis takes place in a plane perpendicular to the axis. 32. Fig. 35 shows the plan and elevation of a circle whichhas been made by revolving the point A about the line MN whichlies in H. From this figure it will be seen that when A lies in H, 38 REVOLUTION OF POINTS 39
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