The Register.—The fixed point in the center of the bar. a.
The Trammel.—The pointer which slides on the bar. b.
The Pointer.—The pointer at the top of the trammel.
The Index.—The lower point on the trammel.
Normals.—The straight vertical lines in each set of curves.
The String.—The cord on the right side of the board used in determining travel.
The Travel-scale.—Scales for setting the string.
Prediction-scale.—Vertical lines on the right side of the board, used in determining the travel during the observing interval.
ADJUSTMENT.
Adjust the ruler by means of the adjusting-screw on the left, so that its upper edge coincides with the parallel lines on the board.