THE HODOMETER
1. The drift of our treatise now turns to a useful invention of the
greatest ingenuity, transmitted by our predecessors, which enables us,
while sitting in a carriage on the road or sailing by sea, to know how
many miles of a journey we have accomplished. This will be possible as
follows. Let the wheels of the carriage be each four feet in diameter,
so that if a wheel has a mark made upon it, and begins to move forward
from that mark in making its revolution on the surface of the road, it
will have covered the definite distance of twelve and a half feet on
reaching that mark at which it began to revolve.
2. Having provided such wheels, let a drum with a single tooth
projecting beyond the face of its circumference be firmly fastened to
the inner side of the hub of the wheel. Then, above this, let a case be
firmly fastened to the body of the carriage, containing a revolving drum
set on edge and mounted on an axle; on the face of the drum there are
four hundred teeth, placed at equal intervals, and engaging the tooth of
the drum below. The upper drum has, moreover, one tooth fixed to its
side and standing out farther than the other teeth.
3. Then, above, let there be a horizontal drum, similarly toothed and
contained in another case, with its teeth engaging the tooth fixed to
the side of the second drum, and let as 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
obstruction is removed, falls with a ringing sound into a bronze vessel.
7. So, when a ship is making headway, whether under oars or under a gale
of wind, the floatboards on the wheels will strike against the water and
be driven violently back, thus turning the wheels; and they, revolving,
will move the axle, and the axle the drum, the tooth of which, as it
goes round, strikes one of the teeth of the second drum at each
revolution, and makes it turn a little. So, when the floatboards have
caused the wheels to revolve four hundred times, this drum, having
turned round once, will strike a tooth of the horizontal drum with the
tooth that is fixed to its side. Hence, every time the turning of the
horizontal drum brings a stone to a hole, it will let the stone out
through the pipe. Thus by the sound and the number, the length of the
voyage will be shown in miles.
I have described how to make things that may be provided for use and
amusement in times that are peaceful and without fear.