368 l):s lieplif to C:s Observations [Mav, 



inches in an hour." — (Mr. IIerapath*s Reply to Mr. Tredgold, 

 Afuiah lor Dec. 1821, p. 4G4.) 



" Had 1 *** it would have been like endeavouring to equate a 

 single impulse with an unceasing force," (pressure he means) 

 ** for an indefinite time — a manifest impossibility." — (Mr. II. 's 

 Reply to X. Aumih for January, 1822, p. 30.) 



C. tells us that the ^^ pushing" case I have just quoted, which 

 (with how much truth tlie reader may judge from the counter 

 quotations), he informs the world, is Mr. Herapath's, is that by 

 w Inch it is intended by Mr. H. that '* the doctrines of Newton, 

 Maclaurin, Hutton, Playfair, and innumerable other mathemati- 

 cians, are to be overturned in relation to the collision of hard 

 bodies." We have already seen that C. has not been over for- 

 tunate in quoting, nor very happy in understanding, the writings 

 of Dr. ilutton and Prof. Playfair, though it be true they dehver 

 *' principles as nearly as possible self-evident." In the present 

 instance, I think, we shall find he has not been more successful 

 in his acquaintance with Sir I. Newton. However, it is neces- 

 sary for me to premise, that I am not aware Newton has said any 

 thing of collision, except in the first part of his Principia. If he 

 has in any other places, I shall be happy to be corrected ; for I 

 do not recollect to have seen it. Even here he has given nothing 

 in the shape of regular argument : a few loose ideas only, thrown 

 in apparently more by accident than design, are all I can per- 

 ceive. They happen, however, to be of that peculiar cast as to 

 satisfy us that C. has either not seen them, or not understood 

 them ; or what, perhaps, is more probable, that his zeal to oppose 

 Mr. Herapath has outstripped his discretion and his knowledge ; 

 and hence occasioned him to quote authors without knowing what 

 they have written. 



In the Schol. Cor. 6, Principia, Third Law of Motion, Newton 

 says : " By the theory of Wren and Huygens, bodies absolutely hard 

 return from one another with the same velocity with which they 

 meet." This was the case with the theories of Wren and Huy- 

 gens when the bodies were supposed to be perfectly equal ; and, 

 therefore, instead of coinciding with the doctrine of collision C. 

 advocates, it coincides with Mr. Herapath's. That Newton was 

 not here confounding hard with elastic bodies appears from the 

 sentence immediately following the above, and implying that 

 though he did not question the truth of this case or Wren and 

 Huygens's theories, yet the evidence of it was greater in elastic 

 bodies. " But this may be affirmed," says Newton, *^ with more 

 certainty in perfectly elastic bodies." 



To satisfy us what his opinion was, he says in the same Schol. 

 '* By the same " (first and second Laws of Motion), '^ together 

 with the third Law, Sir Christopher Wren, Dr. Wallis, and Mr. 

 Huygens, the greatest geometers of our times, did se/erally 

 determine the rules of the congress and reflection of hard bodies 



