SOLAR AND STELLAR PROPER MOTIONS. 161 
in one of the important departments of astronomical science ; 
and at the same time to indicate the progress of that line of 
thought which tended to uproot the theory of the immobility 
of the solar system, and to supplant the speculations of a 
baseless philosophy with a broader theory of the Universe, 
resting on the solid foundation of numerous and well ob¬ 
served facts. 
The progress of science has often been likened to the march 
of a grand army, sweeping with a steady but irresistible mo¬ 
mentum towards some definite point; and, again, it has been 
characterized as the advance of an undisciplined throng, 
without a leader, but finding its directing impetus in the 
common impulse of its individual members. In the' special 
investigation considered to-night, the progress seems to me to 
be more like that of a rising flood invading for the first time 
the surface of a partially broken but unexplored country. 
There is no steady, uniform progress in any direction. When 
the tide overrides the barrier to an unoccupied basin beyond, 
the rush of the current is not stayed until every nook and 
indentation in the fresh fields has been searched out, and the 
new possibilities have been exhausted. Then a period of 
tranquillity ensues, until a higher level is reached, fresh ob¬ 
stacles are surmounted, and the accumulated energy is diverted 
into new regions where exploration is again concentrated, 
until the rising flood is strong enough to break its way into 
still higher areas, intermittingly repeating the cycle of renew¬ 
ing rest and impulsive effort. 
A careful examination of the literature of solar and stellar 
motions will reveal, in a striking manner, the intermittent 
character of the numerous investigations undertaken within 
the past century. New observations and more ingenious 
methods have revived, at irregular intervals and with fresh 
energy, the hope that new light might be thrown upon the 
troublesome problem, and many zealous students have en¬ 
tered the field, at nearly the same, time to search for a more 
satisfactory solution. 
It is now one hundred and seventy years since Halley 
