the pivots, h and k; this bar represents the "ray." A projecting arm k
k' has been introduced to enable me to shorten the polar distance down
to 2 in. and under by removing the pivot, k to k'. m m is a bar
attached to the block, n, which runs on l l, so that m m is always
perpendicular to l l. On the chariot, B, is another bar, m' m',
capable of turning round the pivot, d, and always maintained parallel
to m m by the rods, m m', m m'. Attached to m' m' is a wheel, w,
whose axis is parallel to m' m'. This wheel, therefore, always moves
perpendicular to m' m', and therefore to m m; hence it moves parallel
to the ray, h k. A pencil, P', attached traces out the sum curve. If
we wish to use the machine as an integrator, we have merely to measure
the vertical distance traversed by P', or the distance B has run along
f' f'. This is done by means of a scale on f f'. If k be brought down
to k0, w runs parallel to g g, or P' traces out a horizontal
straight line, which is thus the base line. If k be fixed as near as
possible to k0, which is done by means of a screw in f f at k0,
the chariot, B, can be run down f' f' as nearly opposite to k0 as
can be guessed at; a horizontal line may then be drawn as base line,
and the guide point, P, brought into this line by a clamping screw
with which it is provided. The instrument is then ready for action.
There is a brake on one of the roughed wheels to check or stop the
motion of the integraph when required.

The instrument works best when the chariots, A and B, are about
opposite to each other; when they are at opposite extremities of f f
and f' f' respectively, the pull at P tends to produce a skewing
couple. If the chariot, B, could be put upon f f and work, if needful,
by a double parallelogram from m m, we should have, excepting the skew
pull, some great practical advantages. We might throw the whole of the
weight of the machine on the one pair of friction wheels, and replace