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                          CURVED    LINES.
and expansion of the
 one to know what it is intended for; but to
 draw it in its exact proportions, with the sweep
 of the outline in perfect balance on either side;
 to make it a true representation of the object,
 some method    must be used.    Having fixed
 upon the height of the glass A B, decide upon
 the diameter of its base or stand n c~ and that
 of the top B r. That done, you have sure
 starting points; and nothing more remains, to
 complete the outline, than first determining,
 by your eye, the variation of the curves it
 presents from these right lines, and expressing
 them exactly as you have already done in the
 examples before given (22).  WitI~ the straight
 lines B B-B r to guide you, the gradual taper    ~
object is readily expressed by one clear sweep, easily obtained and repeated.
  30. The first and greatest difficulty of the beginner will be to find and
see these imaginary
straight lines in objects presenting, in their form and outline, only irregular
curves.  This must be
                        acquired by training.  By prac-  A    B         
                 C    12
                        tice and observation, the eye
                        will soon learn to find them out,
                        without mechanical aid.   Let
                        him, as a first experiment, for
                        instance, hold a thread, with a
                        alight weight attached to it, at
                        arm's length, between him and
                        an ordinary water-pitcher, or
                        ewer, and he will at once see
                        all the perpendicular lines he
                        desires, drawn, as it were, against
                        the pitcher by the thread. They
                        will show him the relative van-
                        ations of all the ct~rvatures of
                                   4