axes of the wheels that support and guide them are made thin, and roll
over the plane surface of recesses formed for the purpose in the lateral
steel surfaces of the carriages, while the circumference of the wheels
rolls in grooves along the two T-squares.

These latter are, on the one hand, carried by rollers that run in the
groove of the iron, I, and, on the other, by a single roller that runs
over the paper. At right angles with one of these bars is fixed a
divided ruler, through one point of which continually passes a third
ruler, whose extremity pivots upon the point, A, of the first carriage.

When the divided ruler is placed upon the axis of the X's, and the
point, A, of this carriage is following the contours of the figure to be
integrated, the tangent of the angle made by the inclined ruler with the
axis of the X's will be proportional to the ordinate of the figure. The
wheels, R and R', of the drawing-pen, A', of the second carriage must
move parallel with this ruler. In order to obtain such parallelism, we
employ a parallelogram formed as follows: Two gear-wheels of the same
diameter are fixed upon the ruler that ends at the point, A, of the
first carriage, and their line of centers is parallel with the latter.
The second carriage likewise carries two drums equal in diameter to
those of the toothed wheels. These are fixed, and their line of centers
must remain constantly parallel with the line of centers of the
gear-wheels, and consequently with the straight line which passes
through the point, A. This parallelism is obtained by means of a weak
steel spring, or of a silken thread passing over the four wheels, the
two first of which (the gear-wheels) hold it taut by means of a barrel
and spring placed in the center of one of them.

The edge of the wheels, R, R', of the second carriage prevents the