PRESIDENTIAL ADDRESS—PRIESTLEY. yi 
had come with the improvement in means of observation 
and in the development of mathematical knowledge. ' An 
example of the importance of this progress is found in 
Kepler’s work. Kepler’s laws of planetary motion arose 
from his study of the observations of Mars recorded by 
Tycho Brahe, in whose observatory he was an assistant. 
At the beginning of the work, under the sway of the old 
idea of the perfect circular motion, he attempted to 
deseribe the motion of the planet by means of epicyclics 
and very nearly succeeded. In his own account of his 
investigations we read :-— 
‘“Sinee the divine goodness has given to us Tycho 
Brahe, a most careful observer, from whose observations 
the error of 8’ is shown in this calculation, it is right that 
we should with gratitude recognize and make use of this 
cift of God. For if I could have treated 8’ of longitude 
as negligible I should have already corrected sufficiently 
the hypothesis discovered in Chapter XVI; but as they 
could not be neglected, these 8’ alone have led the way 
towards the complete reformation of Astronomy.”’ 
Thus Kepler was warned off his epicyclic hypothesis 
by a discrepancy of eight minutes between calculated and 
observed results, an amount that could not have been 
detected by any Greek observer. Furthermore, his final 
explanation was in terms of the ellipse, a curve discovered 
by the Greek geometers, certainly, but unknown before 
metaphysics had obtained its hold on Greek Science. 
The fact that he arrived at a law that could not 
have been obtained by a Greek astronomer does not neces- 
sarily imply the superiority of his method, but may have 
arisen from the superiority of the means at his disposal. 
The detailed analysis of observations involved in the 
new scientific method required much description of 
phenomena and thereby caused an all-important change 
in the viewpoint of the scientific world. The question 
‘“Why’’ had dominated the older thought, but emphasis 
now was placed on the question ‘‘How.’’ It is unhkely 
that Newton and his contemporaries had adopted our 
modern view that the business of Science is to give an 
ordered description of phenomena and has no -concern 
with first causes; but it is certain that they realized that 
