If I remember correctly:

The murder in MURDER AT THE SMITHSONIAN took place on the balcony
overlooking the pendulum. The victim is killed during an elegant,
high powered opening; he falls down into the rotunda, landing in the
midst of the swinging pendulum and its time-keeping markers. I'd tell
where the murderer hides, but don't want to spoil the plot for those
who have not yet read this enjoyable little mystery.