INTRODUCTION. 5 



tion of numbers to the physical problems of the universe, and 

 lost themselves in forced analogies and conjecture regarding 

 the "harmony of the spheres." According to Diogenes 

 Laertius, Pythagoras imagined the universe in the form of a 

 sphere. The earth was in the centre, and bore the axis around 

 which the firmament revolved. The moon, the sun, Mercury, 

 Venus, Mars, Jupiter, and Saturn described circular paths 

 round the earth, and the harmonic motion of these bodies 

 called forth the music of the spheres. The Pythagorean 

 Philolaus improved on this conception. He described the 

 universe as a system comprising ten heavenly bodies the five 

 planets, the sun, the moon, the earth, and a counter-earth 

 which moved from west to east round a " central-fire." The 

 earth turned one half towards the central-fire, whilst the other, 

 or inhabited half, received light and heat from the sun. 

 Entirely beyond the circles of this system lay the fixed stars 

 and the illimitable ether from which the universe drew its 

 breath. 



The principle of constant change taught by Pythagoras and 

 Heraclitus is also a leading feature in the doctrines of 

 Empedocles of Agrigentum (492-432 B.C.). Empedocles sup- 

 posed that everything had its origin in, and took its components 

 from, four elements (earth, water, air, and fire) ; that these 

 elements were without beginning and imperishable, but subject 

 to never-ending change. From these elements the world at 

 one time took shape, and it must at some future time be again 

 dispersed. The course of the world's existence resolved itself 

 into a history of recurring periods and phases. As Empedocles 

 did not concern himself about an empirical basis for most of 

 his theories, it is of little avail to enter into his physical and 

 biological speculations. Geology, however, owes one distinct 

 step in advance to this philosopher. Whereas the Pythagoreans 

 had conjectured the presence of a central fire in the universe, 

 Empedocles taught that the earth's centre was composed of 

 molten material. Empedocles formed this opinion on the basis 

 of his actual observation of the volcanic activities of Mount 

 Etna. Tradition says that he met his death by falling into the 

 crater of that volcano. 



Leucippus and Democritus of Abdera (circa 490 B.C.) were 

 the founders of the school of atomic philosophy, which of all 

 the Greek systems approaches most nearly to the opinions of 

 the present day. According to Democritus, the only realities 



