78 HISTORY OF MECHANICS. 



motion 2 , and had distinctly contemplated the pro- 

 blem of the motion of a body in an ellipse by a 

 central force, though they could not solve it. Hal- 

 ley went to Cambridge in 1684 3 , for the express 

 purpose of consulting Newton upon the subject of 

 the production of the elliptical motion of the planets 

 by means of a central force, and, on the 10th of 

 December 4 , announced to the Royal Society that he 

 had seen Mr. Newton's book, De Motu Corporum. 

 The feeling that mathematicians were on the brink 

 of discoveries such as are contained in this work 

 was so strong, that Dr. Halley was requested to 

 remind Mr. Newton of his promise of entering them 

 in the Register of the Society, "for securing the 

 invention to himself till such time as he can be 

 at leisure to publish it." The manuscript, with the 

 title Philosophic Naturalis Principia Mathematical, 

 was presented to the society (to which it was dedi- 

 cated,) on the 28th of April, 1686, Dr. Vincent, 

 who presented it, spoke of the novelty and dignity 

 of the subject ; and the president (Sir J. Hoskins) 

 added, with great truth, "that the method was so 

 much the more to be prized as it was both invented 

 and perfected at the same time." 



The reader will recollect that we are here 

 speaking of the Principia as a Mechanical Treatise 

 only; we shall afterwards have to consider it as 

 containing the greatest discoveries of Physical As- 



2 Newt. Princip. Schol. to Prop. iv. 



3 Sir D. Brewster's Life of Nervt. p. 154. Id. p. 184, 



