he sun alone comes
out about one foot, or let us say one foot for simplicity. Now the
actual tide observed in mid-Atlantic is at the springs about four feet,
at the neaps about two. The spring tide is lunar plus solar; the neap
tide is lunar minus solar. Hence it appears that the tide caused by the
moon alone must be about three feet, when unaffected by momentum. From
this datum Newton made the first attempt to approximately estimate the
mass of the moon. I said that the masses of satellites must be
estimated, if at all, by the perturbation they are able to cause. The
lunar tide is a perturbation in the diurnal motion of the sea, and its
amount is therefore a legitimate mode of calculating the moon's mass.
The available data were not at all good, however; nor are they even now
very perfect; and so the estimate was a good way out. It is now
considered that the mass of the moon is about one-eightieth that of the
earth.

* * * * *

Such are some of the gems extracted from their setting in the
_Principia_, and presented as clearly as I am able before you.

Do you realize the tremendous stride in knowledge--not a stride, as
Whewell says, nor yet a leap, but a flight--which has occurred between
the dim gropings of Kepler, the elementary truths of Galileo, the
fascinating but wild speculations of Descartes, and this magnificent and
comprehensive system of ordered knowledge. To some his genius seemed
almost divine. "Does Mr. Newton eat, drink, sleep, like other men?" said
the Marquis de l'Hopital, a French mathematician of no mean eminence; "I
picture him to myself as a celestial genius, entirely removed from the
restrictions of ordinary matter." To many it seemed as if there was
nothing more to be discovered, as if the universe were now explored, and
only a few fragments of truth remained for the gleaner. This is the
attitude of mind expressed in Pope's famous epigram:--

"Nature and Nature's laws lay hid in Night,
God said, Let Newton be, and all was light."

This feeling of hopelessness and impotence was very natural after the
advent of so overpowering a genius, and it prevailed in England for
fully a century. It was very natural, but it was very mischievous; for,
as a consequence, nothing of great moment was done by England in
science, and no Englishman of the first magnitude appeared, till some
who are either living now or who have lived within the present century.

It appeared to his contemp