Nineteenth and Twentieth Century Clouds Over the Twenty-First Century   Virtual Observatory
  Physical science has changed in the century since Lord Kelvin's celebrated essay on Nineteenth Century Clouds over the Dynamical Theory of Heat and Light, but some things are the same. Analogs in what was happening in physics then and what is happening in astronomy today serve to remind us why we can be confident the Virtual Observatory of the twenty-first century will have a rich list of challenges to explore. 
P. J. E. Peebles