According to the discussion on that web page, this a demonstration of
"conservation of mechanical energy". Forsooth, it demonstrates
nearly-lossless conversion of gravitational potential energy to
kinetic energy and back again.
The curious thing is that Galileo published this in 1638. Apparently
not everybody got the point. Decades later Leibnitz was arguing that
kinetic energy by itself was conserved, and it took the better part
of two centuries for a full understanding of conservation of energy
to emerge.