Final answer:
The second event to occur in the provided timeline is Clara Barton working as the first female federal patent clerk in 1855.
Step-by-step explanation:
Based on the timeline provided regarding Clara Barton's life and achievements, we can establish the order of major events and determine which happened second according to the timeline. The events listed in the timeline, in chronological order, are:
- Barton cares for her brother David after he falls from a roof in 1831.
- Barton moves to Washington D.C. to work as the first female federal patent clerk in 1855.
- Barton begins gathering and distributing supplies during the Civil War in 1861.
- Barton works on the front lines as a self-taught nurse in 1862.
- Barton identifies anonymous soldiers with the Office of Missing Soldiers in 1865.
- Barton travels to Europe in 1869.
From this, we can conclude that the second event to occur was Barton works as a federal patent clerk in 1855.