http://www2.lowell.edu/Research/library/paper/lowell.html
http://www.space.com/19774-percival-lowell-biography.html
http://www.britannica.com/EBchecked/topic/349831/Percival-Lowell
http://www.bbc.co.uk/science/space/solarsystem/scientists/percival_lowell
http://www.discoveryofpluto.com/pluto04.html
