176 A Short History of Astronomy [Cn. VI. 



by its horizontal motion, and consequently it reaches 

 the ground at the same time as a bullet simply allowed 

 to drop; but Galilei gives no general statement of this 

 principle, which was afterwards embodied by Newton in 

 his Second Law of Motion. 



The treatise on the Two New Sciences was finished in 

 1636, and, since no book of Galilei's could be printed in 

 Italy, it was published after some little delay at Ley den 

 in 1638. In the same year his eyesight, which he had 

 to some extent recovered after his first attack of blindness, 

 failed completely, and four years later (January 8th, 1642) 

 the end came. 



134. Galilei's chief scientific discoveries have already 

 been noticed. The telescopic discoveries, on which much 

 of his popular reputation rests, have probably attracted 

 more than their fair share of attention ; many of them 

 were made almost simultaneously by others, and the rest, 

 being almost inevitable results of the invention of the 

 telescope, could not have been delayed long. But the 

 skilful use which Galilei made of them as arguments for 

 the Coppernican system, the no less important support 

 which his dynamical discoveries gave to the same cause, 

 the lucidity and dialectic brilliance with which he marshalled 

 the arguments in favour of his views and demolished 

 those of his opponents, together with the sensational in- 

 cidents of his persecution, formed conjointly a contribution 

 to the Coppernican controversy which was in effect 

 decisive. Astronomical text-books still continued to give 

 side by side accounts of the Ptolemaic and of the Copper- 

 nican systems, and the authors, at any rate if they were 

 good Roman Catholics, usually expressed, in some more 

 or less perfunctory way, their adherence to the former, but 

 there was no real life left in the traditional astronomy ; 

 new advances in astronomical theory were all on Copper- 

 nican lines, and in the extensive scientific correspondence 

 of Newton and his contemporaries the truth of the 

 Coppernican system scarcely ever appears as a subject for 

 discussion. 



Galilei's dynamical discoveries, which are only in part 

 of astronomical importance, are in many respects his 

 most remarkable contribution to science. For whereas in 



