many holes be made in this
(third) drum as will correspond to the number of miles--more or less, it
does not matter--that a carriage can go in a day's journey. Let a small
round stone be placed in every one of these holes, and in the receptacle
or case containing that drum let one hole be made, with a small pipe
attached, through which, when they reach that point, the stones placed
in the drum may fall one by one into a bronze vessel set underneath in
the body, of the carriage.

4. Thus, as the wheel in going forward carries with it the lowest drum,
and as the tooth of this at every revolution strikes against the teeth
of the upper drum, and makes it move along, the result will be that the
upper drum is carried round once for every four hundred revolutions of
the lowest, and that the tooth fixed to its side pushes forward one
tooth of the horizontal drum. Since, therefore, with four hundred
revolutions of the lowest drum, the upper will revolve once, the
progress made will be a distance of five thousand feet or one mile.
Hence, every stone, making a ringing sound as it falls, will give
warning that we have gone one mile. The number of stones gathered from
beneath and counted, will show the number of miles in the day's journey.

5. On board ship, also, the same principles may be employed with a few
changes. An axle is passed through the sides of the ship, with its ends
projecting, and wheels are mounted on them, four feet in diameter, with
projecting floatboards fastened round their faces and striking the
water. The middle of the axle in the middle of the ship carries a drum
with one tooth projecting beyond its circumference. Here a case is
placed containing a drum with four hundred teeth at regular intervals,
engaging the tooth of the drum that is mounted on the axle, and having
also one other tooth fixed to its side and projecting beyond its
circumference.

6. Above, in another case fastened to the former, is a horizontal drum
toothed in the same way, and with its teeth engaging the tooth fixed to
the side of the drum that is set on edge, so that one of the teeth of
the horizontal drum is struck at each revolution of that tooth, and the
horizontal drum is thus made to revolve in a circle. Let holes be made
in the horizontal drum, in which holes small round stones are to be
placed. In the receptacle or case containing that drum, let one hole be
opened with a small pipe attached, through which a stone, as soon as the