$$128,129] The Dialogue on the Tivo Chief Systems 163 



more and more doubtful about the book. Ultimately, 

 however, the introduction and conclusion having been sent 

 to Rome for approval and probably to some extent re- 

 written there, and the whole work having been approved 

 by the Florentine censor, the book was printed and the 

 first copies were ready early in 1632, bearing both the 

 Roman and the Florentine imprimatur. 



129. The Dialogue extends over four successive days, 

 and is carried on by three speakers, of whom Salviati is a 

 Coppernican and Simplicio an Aristotelian philosopher, 

 while Sagredo is avowedly neutral, but on almost every 

 occasion either agrees with Salviati at once or is easily 

 convinced by him, and frequently joins in casting ridicule 

 upon the arguments of the unfortunate Simplicio. Though 

 many of the arguments have now lost their immediate 

 interest, and the book is unduly long, it is still very read- 

 able, and the specimens of scholastic reasoning put into 

 the mouth of Simplicio and the refutation of them by 

 the other speakers strike the modern reader as excellent 

 fooling. 



Many of the arguments used had been published by 

 Galilei in earlier books, but gain impressiveness and cogency 

 by being, collected and systematically arranged. The 

 Aristotelian dogma of the immutability of the celestial 

 bodies is once more belaboured, and shewn to be not 

 only inconsistent with observations of the moon, the sun, 

 comets, and new stars, but to be in reality incapable of 

 being stated in a form free from obscurity and self-con- 

 tradiction. The evidence in favour of the earth's motion 

 derived from the existence of Jupiter's satellites and from 

 the undoubted phases of Venus, from the suspected phases 

 of Mercury and from the variations in the apparent size of 

 Mars, are once more insisted on. The greater simplicity 

 of the Coppernican explanation of the daily motion of the 

 celestial sphere and of the motion of the planets is forcibly 

 urged and illustrated in detail. It is pointed out that on 

 the Coppernican hypothesis all motions of revolution or 

 rotation take place in the same direction (from west to 

 east), whereas the Ptolemaic hypothesis requires some 

 to be in one direction, some in another. Moreover the 

 apparent daily motion of the stars, which appears simple 



