http://digital.csic.es/bitstream/10261/7493/2/Dynamical_mech...
http://www.haskins.yale.edu/Reprints/HL1521.pdf
http://digital.csic.es/bitstream/10261/7493/2/Dynamical_mech...
http://www.haskins.yale.edu/Reprints/HL1521.pdf