Excerpt:
In 1845, at the request of the Secretary of the Navy, the Observatory installed a time ball atop the 9.6-inch telescope dome. The time ball was dropped every day precisely at Noon, enabling the inhabitants of Washington to set their timepieces. Ships in the Potomac River could also set their clocks before putting to sea. The Observatory's Time Service was initiated in 1865. A time signal was transmitted via telegraph lines to the Navy Department, and also activated the Washington fire bells at 0700, 1200, and 1800. This service was later extended via Western Union telegraph lines to provide accurate time to railroads across the nation. The Observatory participated in a program of determining longitude by comparing local time with that telegraphed from a clock at another fixed observatory, and thus exchanged time signals with other observatories and with the Coast Survey field parties.