EVOLUTION OF THE STELLAR SYSTEMS. 235 
to be unsound or incomplete. That tidal oscillations which were first noticed by the 
navigators of our seas are at length seen to be but special phenomena of a general law 
operating throughout the universe is alike honorable and gratifying to the human mind. 
It is equally inspiring to recall that by the known laws of these phenomena we are 
enabled to trace existing systems through immeasurable time, and thus disclose cosmical 
history which mortal eye could never witness. In our time it is no longer sufficient to 
maintain the traditions of the past, to trace the planets, satellites and comets through 
centuries, and explain observed anomalies in their figures, attractions and orbital motions 
by the law of gravitation. We-must essay to discover the cosmical processes by which 
the existing order of things has come about. Though it seems probable that a fair begin- 
ning on this problem has already been made, a much greater work remains to be done 
during this and the coming century. 
What is needed is a more thorough exploration of the face of the heavens, by 
astronomers who are familiar with the laws of mechanics; and a far-reaching investiga- 
tion of the general theory of tides in viscous liquid and gaseous masses such as the stars 
and nebule of remote space. Even if the full extent of the hopes here expressed can be 
realized only after the lapse of several centuries, I venture to believe that the achievement 
will not be unworthy of the past history of Physical Astronomy. 
