ave
had to the same extent; because the lives and energies of these great
men would have been partially consumed in obtaining the main facts
themselves.

The youngest of the three questioners at the time we are speaking of was
Edmund Halley, an able and remarkable man. He had been at Cambridge,
doubtless had heard Newton lecture, and had acquired a great veneration
for him.

In January, 1684, we find Wren offering Hooke and Halley a prize, in the
shape of a book worth forty shillings, if they would either of them
bring him within two months a demonstration that the path of a planet
subject to an inverse square law would be an ellipse. Not in two months,
nor yet in seven, was there any proof forthcoming. So at last, in
August, Halley went over to Cambridge to speak to Newton about the
difficult problem and secure his aid. Arriving at his rooms he went
straight to the point. He said, "What path will a body describe if it
be attracted by a centre with a force varying as the inverse square of
the distance." To which Newton at once replied, "An ellipse." "How on
earth do you know?" said Halley in amazement. "Why, I have calculated
it," and began hunting about for the paper. He actually couldn't find it
just then, but sent it him shortly by post, and with it much more--in
fact, what appeared to be a complete treatise on motion in general.

With his valuable burden Halley hastened to the Royal Society and told
them what he had discovered. The Society at his representation wrote to
Mr. Newton asking leave that it might be printed. To this he consented;
but the Royal Society wisely appointed Mr. Halley to see after him and
jog his memory, in case he forgot about it. However, he set to work to
polish it up and finish it, and added to it a great number of later
developments and embellishments, especially the part concerning the
lunar theory, which gave him a deal of trouble--and no wonder; for in
the way he has put it there never was a man yet living who could have
done the same thing. Mathematicians regard the achievement now as men
might stare at the work of some demigod of a bygone age, wondering what
manner of man this was, able to wield such ponderous implements with
such apparent ease.

To Halley the world owes a great debt of gratitude--first, for
discovering the _Principia_; second, for seeing it through the press;
and third, for defraying the cost of its publication out of his own
scanty purse. For though he ultimate