President's Address. 
47 
imagination in a higher degree than other sciences is that astronomy is 
par excellence the science of prediction. True, the days are now past 
when an astronomer is regarded, except by the most ignorant, as gifted 
with supernatural powers and capable of predicting events that can have 
no conceivable relationship with the objects of his researches, or when an 
unscrupulous astronomer could utilise his powers of prediction for imposing 
on the world at large in the face of the criticisms of fellow-workers in 
collateral branches of science. Nevertheless it is only necessary to point 
to any of the leading almanacs to establish the undoubted claim of 
astronomy to a considerable predictive capacity in its own legitimate 
sphere. These almanacs, prepared in advance, give from day to day the 
positions of the sun and moon, the phenomena of eclipses and various 
other data with an accuracy which can only be called in question by the 
most refined tests available to astronomers. 
How, then, has astronomy acquired this faculty ? The answw to this 
question is — at least primarily — by continuous and patient observation, using 
always the most refined methods of physical measurement available. 
A well-devised scheme of observation is sooner or later bound to lead 
to the detection of laws governing physical phenomena if such laws exist. 
Thus it was the planetary observations of Tycho Brahe which led to the 
detection of the laws of planetary motion associated with the name of 
Kepler. 
Once such laws have been established and the necessary initial data 
secured, the science of astronomical prediction would for the future 
devolve on the mathematician rather than the astronomer, were it 
not for two sources of uncertainty with which the astronomer must 
continue to concern himself. It is evident, on the one hand, that we 
cannot infuse into our predicted phenomena greater precision than that 
derived from the initial data, themselves dependent on imperfect observa- 
tions. However well the laws governing planetary motions may be 
understood, the predicted position of a planet to-day depends on its 
observed positions at some earlier epoch or epochs ; and the fallibility of 
the observations made at these earlier epochs will not only pervade all 
future predictions, but will inevitably increase in amount as the epoch 
of prediction recedes from the epoch of observation. For this reason, if 
the standard of accuracy of prediction is to be merely maintained — and 
the growing requirements of science will scarcely rest contented with this 
— continuous observation must be maintained and the data on which pre- 
dictions are based revised from time to time. 
I have dealt so far only with the effects of the unavoidable inaccuracies 
of observations, even when pushed to their utmost refinement, as 
influencing results of prediction. A second consideration of even greater 
importance is the validity of the laws associating the predicted with the 
