SEQUEL TO THE EPOCH OF GALILEO. 341 



professor at Paris ; Wallis, who was appointed Savilian professor at 

 Oxford in 1649, his predecessor being ejected by the parliamentary 

 commissioners. It is not necessary for us to trace the progress of 

 purely mathematical inventions, which constitute a great part of the 

 works of these authors ; but a few circumstances may be mentioned. 



The question of the proof of the Second Law of Motion was, from 

 the first, identified with the controversy respecting the truth of the Co- 

 pernican System ; for this law supplied the true answer to the most 

 formidable of the objections against the motion of the earth ; namely, 

 that if the earth were moving, bodies which were dropt from an ele- 

 vated object would be left behind by the place from which they fell. 

 This argument was reproduced in various forms by the opponents of 

 the new doctrine ; and the answers to the argument, though they be- 

 long to the history of Astronomy, and form part of the Sequel to the 

 Epoch of Copernicus, belong more peculiarly to the history of Mechan- 

 ics, and are events in the sequel to the Discoveries of Galileo. So far, 

 indeed, as the mechanical controversy was concerned, the advocates 

 of the Second Law of Motion appealed, very triumphantly, to exper- 

 iment. Gassendi made many experiments on this subject publicly, of 

 which an account is given in his Epistolce tres de Motu Impresso a 

 Motore Translate? It appeared in these experiments, that bodies let 

 fall downwards, or cast upwards, forwards, or backwards, from a ship, 

 or chariot, or man, whether at rest, or in any degree of motion, had 

 always the same motion relatively to the motor. In the application 

 of this principle to the system of the world, indeed, Gassendi and 

 other philosophers of his time were greatly hampered ; for the deference 

 which religious scruples required, did not allow them to say that the 

 earth really moved, but only that the physical reasons against its mo- 

 tion were invalid. This restriction enabled Riccioli and other writers 

 on the geocentric side to involve the subject in metaphysical difficul- 

 ties ; but the conviction of men was not permanently shaken by these, 

 and the Second Law of Motion was soon assumed as unquestioned. 



The Laws of the Motion of Falling Bodies, as assigned by Galileo, were 

 confirmed by the reasonings of Gassendi and Fermat, and the experi- 

 ments of Riccioli and Grimaldi ; and the effect of resistance was point- 

 ed out by Marsenne and Dechales. The parabolic motion of Projectiles 

 was more especially illustrated by experiments on the jet which spouts 

 from an orifice in a vessel full of fluid. This mode of experimenting 



2 Mont. ii. 190. 



