This is a "Time Ball"

A small time ball that was used around 1855. This clock would sit in a railroad station or store window and was connected to a telegraph line. At precisely 1:00 P.M. daily, the Royal Observatory at Greenwich would send out a signal to all of these clocks and the ball would drop. By charging for this service, observatories could profit from keeping time.

The ball we see dropped on New Year's Eve every year originated from the practice of observatories dropping a large ball from the top of the building to indicate to ships and towns the precise time each day.

Text and photo from What is it?, via Neatorama. More at Wikipedia, which notes that there are about sixty time balls still in existence.

Reposted from 2010 because I found a longread discussion of time balls (pdf).