(b) A plan or top view means the same thing no matter whether the drawing is in the first or third angle; similarly, an elevation or front view is the same figure for either first or third angle.
(c) For any given object, upper edges or surfaces are visible in plan, and edges or faces on the front are visible in elevation.
(d) A line perpendicUlar to a plane of projection appears on that plane simply as a point.
(e) A line is shown in its true length in any view, only when the line is parallel to the plane on which the view is drawn.
(f) A plane surface is shown in its true size, only when it is parallel to a plane of projection.
(g) A surface perpendicular to a plane of projection is seen edgewise on that plane, and appears as a single line.
(h) When one line is revolved about another line as axis, each point in the first line moves in a circle whose center is on the axis, and the plane of the circle is per pendicular to the axis.
(i) The length of a given line, or the size of a given surface, as seen in projection, is determined wholly by the angle which the line or surface makes with the plane of projection; the greater the angle made with the plane, the less the size of the projection.
74. It will be found that the exercises given below will, if worked out, afford helpful practice in applying the principles of projection.
Exercises in Projection Exercise 1—Place in First Angle. Draw the plan and elevation of a square prism, with its edges vertical, and with two of its faces at 30 degrees with the vertical plane. The base of the prism is 2 inches square, and the length is 4 inches. Show a circular hole 1 inch in diam eter bored through from one end to the other.
NOTE—Two positions are possible, for the face at 30 degrees with V may be inclined either to the left or to the right.
Exercise 2—Third Angle. Draw the plan and elevation of a prism of the same shape and size as in Exercise 1; but place the prism per pendicular to V, and make two faces incline at 60 degrees with H. Show the hole in the prism as before.
is 3 inches in diameter, and the altitude is 4 inches. Locate the base 1/2 inch below the H plane, and the center of the base inches from V.
Exercise 6—Third Angle. Draw the plan and the elevation of a pile of three blocks placed as follows : each one of the three blocks lies on its widest face; all of the blocks have their cor responding edges parallel; the lowest block is on the H plane; the next higher one is placed in the center of the top of the lowest one; and the top block is in the center of the second. All three blocks are rectangular, and have their long edges inclined at 60 degrees with V, back ward and toward the left. The bottom block is 5 inches by inches by inch; the middle one is 4 inches by inches by 1/, inch; and the top one is 3 inches by 21/2 inches by inch.
Exercise 7—Construct a profile view of the blocks in Exercise 6, looking from the right.
Exercise 8—First Angle. Construct the plan and elevation of a rectangular prism 4 inches by 2 inches by inches. The long edges are parallel to both H and V. Place the prism so that the lower back face makes 30 de grees with H.
Suggestion—The end or profile view must be drawn first.
NOTE—Two solutions are possible, since the lower back face may be taken either as the wide or the nar row one.