The text accompanying the diagram (translation by Alexander Ellis (1895)) reads:
"Hence the motion of the string may be briefly thus described. In fig. 63 the foot d of the ordinate of its highest point moves backwards and forwards with a constant velocity on the horizontal line ab, while the highest point of the string describes in succession the two parabolic arcs ac1b and bc2a, and the string itself is always stretched in the two lines ac1 and bc1 or ac1 and bc2."
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