APRIL 5, 1901.] 
to-night to recall some of the principal 
events of these epochs, and to enforce, as 
wellas I may, the great lesson they seem to 
teach us, namely, that science can be main- 
tained only, and can be advanced only, by 
a constant appeal to observation and ex- 
periment. 
As we look out on the universe about us 
the most striking phenomena visible are 
those which belong to what Galileo and his 
successors have fitly called ‘the system of 
the world.’ The rising and setting of the 
sun and moon; the majestic procession of 
the seasons ; the splendid array of the stars 
in the heavens; the ebb and flow of the 
sea, and the never-ending variety from 
wind and weather, need only to be men- 
tioned to enable us to understand why 
astronomy is at once the oldest and one of 
the most highly developed of the sciences. 
No classes of phenomena are so obvious, so 
omnipresent and so enduring. They have 
furnished the symbols of continuity and per- 
manence for all languages in all historic 
times. The ‘fixed stars,’ for example, are 
in fact, as well as in fiction, our standards of 
reference in the reckoning of time and 
space; for are not ‘Sirius and Orion and 
the Pleiades,’ as Carlyle has remarked, 
‘still shining young and clear in their 
course aS when the shepherds first noted 
them on the plains of Shinar ?’ 
But before astronomy there were my- 
thology and astrology, and we may well 
marvel how it has been possible, even after 
the lapse of twenty odd centuries, to educe 
the orderly precision of science out of the 
complicated miscellany of fiction, fact, 
religion, and politics bequeathed to our era 
by the fertile imaginations of our distin- 
guished ancestors. What, for example, 
could be more confusing than the paleon- 
tological jungle called the stellar constella- 
tions, with its gods and godesses ; with its 
dogs, lions, bears and fish, great and small, 
SCIENCE. 
523 
northern and southern ; with its horse, 
whale and goat; and with the slimy forms 
of serpents intertwining them all? 
Although it is impossible to set any date 
for the emergence of astronomy out of 
mythology and astrology, the epoch of 
Hipparchus undoubtedly is the earliest one 
of conspicuous advances known tous. This 
epoch, which may be called also the epoch 
of the Alexandrian school of science, 
extends from about 300 B.c. to about 150 
A.D. It is distinguished by the remark- 
ably perfect work in pure geometry of 
Euclid and Apollonius, and by the still 
more noteworthy work of Archimedes in 
laying the foundations of statics and 
hydrostatics; it comprises the measure- 
ments according to correct principles of the 
obliquity of the ecliptic and the dimensions 
of the earth by Eratosthenes ; it includes 
the observations of the sun, moon, stars 
and planets collected by Aristyllus and 
Timocharis and later turned to so good 
account by Hipparchus ; it embraces the 
work of Aristarchus, who maintained the 
heliocentric theory of the solar system and 
who was the first to attempt a measure of 
the dimensions of that system by means of 
the fine fact of observation that the earth, 
sun and moon form a right triangle, with 
the right angle at the moon when the latter 
is in dichotomy—or when its face is just 
half illuminated ; and finally it includes 
the work of Ptolemy, a worthy disciple of 
Hipparchus, whose Almagest has come 
down to our own time. 
From the observational point of view we 
must rank the principles with respect to 
fluids at rest discovered by Archimedes as 
amongst the capital contributions to the 
science of all times; for while his succes- 
sors, of the last two centuries especially, 
have added to hydromechanics the large 
and vastly more difficult branch of hydro- 
kinetics, they have found no change essen- 
tial in his laws of hydrostatics. 
