And why do you have that obscure piece of history committed to memory? 
Memory? What's that?

In short, two or three days ago I did some reading on VPs through history, how they were and are elected (article II, 12th and 25th amendment), etc, and since my home library is a bit short on material, I made good use of
http://www.loc.gov and
http://www.gutenberg.org. One click lead to another and somehow I ended up with Andrew Johnson, Lincoln's assassination and Booth.
Guess I get distracted too easily...