APRIL 5, 1901.] © 
observation must decide which of them is to 
be preferred. 
The method which Hipparchus used to 
measure the sun’s apparent motion among 
the fixed stars is very noteworthy, especial- 
ly when we consider the utter lack of ef- 
fective instruments in his time. If thesun 
moves regularly about the earth, as first sup- 
posed by Hipparchus, it ought to return at 
any epoch, as that of an equinox, to the same 
position among the fixed stars. Imagine a 
line drawn at the time of the vernal equi- 
nox, say, from the center of the earth to the 
center of the sun. This line prolonged will 
pierce the celestial sphere in two points, 
and, if either point can be located, the posi- 
tion of the sun with reference to the stars 
becomes known. MHipparchus fixed this 
position by noting the location among the 
stars of the center of the shadow cast by 
the earth at the times of eclipses of the 
moon. By a comparison of his own obser- 
vations of such eclipses with those made by 
his predecessors he was able to determine 
the apparent motion of the sun with refer- 
ence to the stars, or what we now know to 
be the motion of the equinoxes with refer- 
ence to stars. To establish this fact of pre- 
cession from such meager observations was 
a great step; and it seems not a little 
singular that a phenomenon so striking 
should not have led to speedy investigations 
for its source. But abouteighteen centuries 
elapsed before Newton clearly visualized 
the mechanical interpretation of this phe- 
nomenon, and it was only after an additional 
half-century that the interpretation was 
fully worked out by d’ Alembert. 
How rapidly the spirit of science dies out 
when its devotees cease to observe and 
experiment is shown by the failure of the 
‘Divine School of Alexandria’ to maintain 
the high standard set by Hipparchus. His 
immediate successors became at best only 
commentators. They wrote much but ob- 
served little; and it does not appear that 
SCIENCE. 
525 
any of them attempted even to verify the 
remarkable discoveries of Hipparchus dur- 
ing the two hundred and fifty years which 
elapsed between the period of his activity 
and the advent of his worthy disciple and 
expounder Ptolemy. 
It is to the work of Ptolemy chiefly that 
we owe our knowledge of the discoveries 
and theories of the Hipparchian epoch. 
His treatise on the ‘Great Construction,’ 
the Megiste Syntaxis, or the Al Magisti and 
hence Almagest of the Arabians, is the 
earliest of the great systematic treatises on 
astronomy. Itisin this work that the theory 
of eccentrics and epicycles of Hipparchus 
is explained and elaborated, and it is this 
work which has given the name of Ptolemy, 
rather than that of his acknowledged mas-: 
ter, to a system of. the world which domi- 
nated scientific thought for nearly fifteen 
hundred years. 
The period during which the observations 
and researches of Ptolemy were carried 
on is commonly referred to in history as 
extending from the reign of the Emperor 
Hadrian to that of Marcus Aurelius. Thus, 
while Ptolemy was an Hygyptian by birth, 
the fact that he was permitted to pursue 
his astronomical studies under the empire 
helps to some extent to relieve the Romans 
of the charge that they were, as regards 
science, the most ignorant people of antiq- 
uity. But the gravity of that charge is 
only palliated by the work of Ptolemy, for 
he left no successors. Roman astronomy 
did not rise above the level of astrology ; 
the spirit of scientific enquiry gave way 
to speculation and declamation; and the 
long night which followed was not broken 
until the dawn of the epoch of Galileo—the 
modern epoch, whose advances have been 
founded on observation and experiment. 
If astronomy is preeminent among the 
sciences for its dependence on observation, 
chemistry and physics are equally preemi- 
