ay be made to move in certain curves so as to be isochronous. Plans
of this kind are employed rather to drive telescopes, phonographs and
other machines requiring uniform and steady movement.

Comical and performing clocks were very popular in the 15th and 16th
centuries. One at Basel in Switzerland was arranged so as gradually to
protrude a long tongue as the pendulum vibrated. It is still to be seen
there in the museum. The famous clock at Strassburg, originally
constructed in 1574, remade in 1842, displays a whole series of scenes,
including processions of the apostles and other persons, and a cock that
crows. A fine clock at Venice has two rather stiff bronze giants that
strike the hours.

Clocks with complicated movements representing the positions of the
heavenly bodies and the days of the week and month, allowance being made
for leap year, were once the delight of the curious. Repeating clocks,
which sounded the hours when a string was pulled, were once popular. The
string simply raised the lifting piece and let the clock strike as the
hands would do when they came to the hour. This was of use in the old
days when the only mode of striking a light at night was with a flint
and steel, but lucifer matches and the electric light have rendered
these clocks obsolete.

[Illustration: FIG. 34.--Curve of Variation of daily rate.]

_Testing Clocks._--The average amount by which a clock gains or loses is
called its mean or average daily rate. A large daily rate of error is no
proof that a clock is a bad one, for it might be completely removed by
pendulum adjustment. What is required is that the daily rate shall be
uniform, that is, that the clock shall not be gaining (or losing) more
on one day than on another, or at one period of the same day than at
another. In fig. 34 A B is a curve in which the abscissae represent
intervals of time, the ordinates the number of seconds at any time by
which the clock is wrong. The curve C D is one in which the ordinates
are proportional to the tangents of the angles of inclination of the
curve A B to the axis of x, that is dy/dx. Whenever the line A B is
horizontal, C D cuts the axis of x. In a clock having no variation in
its daily rate the curve A B would become a straight line, though it
might be inclined to the axis of x, and C D, also a straight line, would
be parallel to the axis of x, though it might not coincide with it. In a
clock set to exact time and having no variations of