cts the divisor once. To count the number of turns of the handle a
second set of windows is arranged with number disks below. These have no
carrying arrangement, but one is turned one step for each turn of the
handle. The machine described is essentially that of Thomas of Colmar,
which was the first that came into practical use. Of earlier machines those
of Leibnitz, Mueller (1782), and Hahn (1809) deserve to be mentioned (see
Dyck, _Catalogue_). Thomas's machine has had many imitations, both in
England and on the Continent, with more or less important alterations.
Joseph Edmondson of Halifax has given it a circular form, which has many
advantages.

The accuracy and durability of any machine depend to a great extent on the
manner in which the carrying mechanism is constructed. Besides, no wheel
must be capable of moving in any other way than that required; hence every
part must be locked and be released only when required to move. Further,
any disk must carry to the next only after the carrying to itself has been
completed. If all were to carry at the same time a considerable force would
be required to turn the handle, and serious strains would be introduced. It
is for this reason that the B-wheels or cylinders have the greater part of
the circumference free from teeth. Again, the carrying acts generally as in
the machine described, in one sense only, and this involves that the handle
be turned always in the same direction. Subtraction therefore cannot be
done by turning it in the opposite way, hence the two wheels C and C' are
introduced. These are moved all at once by one lever acting on a bar shown
at R in section (fig. 2).

In the Brunsviga, the figure disks are all mounted on a common horizontal
axis, the figures being placed on the rim. On the side of each disk and
rigidly connected with it lies its A-wheel with which it can turn
independent of the others. The B-wheels, all fixed on another horizontal
axis, gear directly on the A-wheels. By an ingenious contrivance the teeth
are made to appear from out of the rim to any desired number. The carrying
mechanism, too, is different, and so arranged that the handle can be turned
either way, no special setting being required for subtraction or division.
It is extremely handy, taking up much less room than the others. Professor
Eduard Selling of Wuerzburg has invented an altogether different machine,
which has been made by Max Ott, of Munich. The B-wheels are replaced by