[Footnote 17: Plutarch: _De Facie in Orbe Lunæ_, (V, 410).] Furthermore, one of the weaknesses of the Greek civilization militated strongly against the acceptance of this hypothesis so contrary to the evidence of the senses. Experimentation and the development of applied science was practically an impossibility where the existence of slaves made manual labor degrading and shameful. Men might reason indefinitely; but few, if any, were willing to try to improve the instruments of observation or to test their observations by experiments. At the same time another astronomical theory was developing which was an adequate explanation for the phenomena observed up to that time.[18] This theory of epicycles and eccentrics worked out by Apollonius of Perga (c. 225 B.C.) and by Hipparchus (c. 160 B.C.) and crystallized for posterity in Ptolemy's great treatise on astronomy, the _Almagest_, (c. 140 A.D.) became the fundamental principle of the science until within the last three hundred years. The theory of the eccentric was based on the idea that heavenly bodies Following circular orbits revolved around a center that did not coincide with that of the observer on the earth. That would explain why the sun appeared sometimes nearer the earth and sometimes farther away. The epicycle represented the heavenly body as moving along the circumference of one circle (called the epicycle) the center of which moves on another circle (the deferent). With better observations additional epicycles and eccentric were used to represent the newly observed phenomena till in the later Middle Ages the universe became a "----Sphere With Centric and Eccentric scribbled o'er, Cycle and Epicycle, Orb in Orb"--[19] [Footnote 18: Young: 109.] [Footnote 19: Milton: _Paradise Lost_, Bk. VIII, ll. 82-85.] Yet the heliocentric theory was not forgotten. Vitruvius, a famous Roman architect of the Augustan Age, discussing the system of the universe, declared that Mercury and Venus, the planets nearest the sun, moved around it as their center, though the earth was the center of the universe.[20] This same notion recurs in Martianus Capella's book[21] in the fifth century A.D. and again, somewhat modified, in the 16th century in Tycho Brahe's conception of the universe. [Footnote 20: Vitruvius: _De Architectura_, Lib. IX, c. 4 (220).] [Footnote 21: Martianus Capella: _De Nuptiis_, Lib. VIII, (668).]