> Why? Pendulum clocks (same basic technology) are at least an order of
> magnitude more accurate than that (1 min/day or better).

Because the simple metronomes I was referring to were far from pendulum
clocks.  The ones I remember had an inverted pendulum (pivot at the
bottom).  A spring provided just enough force to keep the pendulum
straight up against gravity.  A normal clock mechanism would cause the
pendulum to oscillate.  A weight with a spring clip was slid up and down
the pendulum to adjust the speed.  Higher weight position increased length
from pivot to center of mass, which reduced the tick rate.  The pendulum
was marked with a beats/minute scale so you knew where to set the weight.
The whole thing was powered by a windup spring, like a windup clock.

I seriously doubt that the spring alone was guaranteed within 1%.
Additional manufacturing variations in the weight, marking of the pendulum
scale, effect of windup tension, etc, very likely exceeded 1% too.

My original point was that nobody at the time complained about the
accuracy of these things.  It just didn't matter whether you practised a
piece at 100 beats/minute or 101 beats/minute.


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