the elbows, raising the bottoms within the
cylinders by repeated and violent blows, and stopping the openings above
by means of the cymbals, compress the air which is enclosed in the
cylinders, and force it into the pipes, through which it runs into the
regulator, and through its neck into the windchest. With a stronger
motion of the levers, the air is still more compressed, streams through
the apertures of the cocks, and fills the channels with wind.

6. So, when the keys, touched by the hand, drive the sliders forward and
draw them back regularly, alternately stopping and opening the holes,
they produce resonant sounds in a great variety of melodies conforming
to the laws of music.

With my best efforts I have striven to set forth an obscure subject
clearly in writing, but the theory of it is not easy, nor readily
understood by all, save only those who have had some practice in things
of this kind. If anybody has failed to understand it, he will certainly
find, when he comes to know the thing itself, that it is carefully and
exquisitely contrived in all respects.

CHAPTER IX

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