Address by Sir David Gill. 23 



from the known distance of the Moon in terms of the Earth's raihus 

 and the rate of fall of heavy objects at the Earth's surface. 



Still, Newton's work was not yet done — in fact in this matter it 

 was only begun. He had not only to conceive all the possible con- 

 sequences of the existence of his law of gravity but to invent the 

 mathematical processes by which these consequences had to be 

 traced. It was only by the combination of the most supreme genius 

 both as a physicist and mathematician, coupled with the severest 

 abstraction of thought from any subject but the one in contemi)latit)n, 

 that such results could be reached ; or, as he himself puts it. " I keep 

 the subject constantly before me till the first dawnings open slowly 

 bv little and little into a full and clear light." 



The vears of 1685 and 1686 will ever be memorable in the 

 history of Science, as in them was produced Newton's Principia. of 

 which it is no exaggeration to say that it is the greatest intellectual 

 effort ever achieved by man. 



It is a notworthy fact that Newton's Principia, which appeared 

 in 1687, was not printed at the cost of the Royal Society, but 

 actually at the private cost of his friend Halley, because that learned 

 but then impecunious body, the Royal Society, had exhausted its 

 resources in the publication of a Historv of Fishes bv a Mr. 

 Willoghby ! 



Some apology is due for introducing in an address like the 

 present so old and oft repeated a story as the history of the discovery 

 of the law of gravitation. My only excuse is that I know of none 

 other which illustrates so completely the points on which I desir,? 

 to insist. 



First of all, note the long laborious process and the efforts of 

 many men and many minds by which alone knowledge of any law 

 • of Nature can be evolved and established, and how great scientific 

 discoveries, like coming events, cast their shadows before. 



Leonardo da Vinci, a hundred and fifty years before Newton, 

 and later, Galileo and Huvgens, all had glimpses of the action of 

 some force like gravity, and wrote about and discussed its existence ; 

 Kepler had the matter almost in his grasp, and probably, but for a 

 preconceived hypothesis that an attractive force must vary as the 

 distance, would have discovered it, although he was not mathe- 

 matician enough to trace out its consequences. Tycho Brahe, 100 

 years before Newton, began the series of observations of the planets 

 which he and Kepler continued for 50 years, when the latter derive.! 

 from their discussion his laws of planetary motion — the primarv key 

 to Newton's discovery. 



Note also Newton's true philosophic spirit. There was no 

 attempt on his part to claim for his hypothesis " a truth of a 

 superior kind to the facts." or to explain away facts, as he might have 

 done in view of the then uncertain determination of the dimensions 

 of the Earth. He waited 1 1 years for improved facts, and then 

 devoted 7 years to the completion of his Principia — the irrefrangible 

 proof of the truth of his theory. 



