10 PROCEEDINGS OF THE ROYAL SOCIETY OF QUEENSLAND. 
Kepler in concentrating on one planet; Mars, before 
attempting to describe the motion of all. The law of 
attraction derived from the simplified problem could be 
nothing more than a tentative suggestion. This was 
realized by its authors, who attempted to establish it more 
firmly by mathematical proof that under such a law and 
such a law only could Kepler’s Laws be true. In this 
attempt they evidently failed, for im August 1684 Halley 
visited Cambridge to consult Newton on the subject. 
The particular question to which he wanted an answer 
was, ‘‘ What is the path of a planet under the inverse square 
law?’? Newton kad already attacked the problem and 
promised to send him a proof of the fact, discovered m 
1679, that the path is an ellipse. On receiving this docu- 
ment in November, Halley was so impressed with its 
importance that he made a special journey to Cambridge 
to persuade Newton to attack the whole problem of gravi- 
tation and to publish the results. The outcome was the 
publication of the Principia in 1687. 
I have given you this brief summary of the history of 
the Principia because it provides an excellent illustration 
of scientific method. We might notice again the main 
features: the careful accumulation of data by Tycho Brahe 
followed by the analysis and concise summary of the facts 
by Kepler; the realization of the possible connection 
between the fall of bodies to the earth and the planetary 
motions; the framing of a tentative hypothesis suggested 
by a simplified problem, and finally a verification of the 
hypothesis by working out its implications and comparing 
them with the results of observation. 
Before leaving this page of the history of Science, I 
would call your attention to the part played by Halley. 
Edmund Halley is generally known from his prediction 
of the return of the comet which bears his name; his great 
claim to the gratitude of the scientific world lies in his 
labours in connection with the Principia. Newton was 
notoriously reluctant to publish his work, and it is to Halley 
that the Principia owes its existence, as is shown in 
Newton’s own preface— 
“In producing this Edmund Halley, that man of 
great intellect and learning, laboured most earnestly. Not 
only did he correct the proofs and get the type engraved, 
