National Republican., 1873-11-29
xony Rooa WANTED,
THE THEORY OF GRAyITATION.
Prof Neweomb's Fourih Lecture.

PrOf NewcOmb's Fourih Lecture.
The fourth lecture Of the course on astronomy
before the Columbian University was delivered
last night by Prof. Ncwcomb, the subject being
"What Power Keeps the Planets In their Orbits. -
Il was reserved for Newton to show that this
mysterious power was none other than what we
see In action whenever an apple falls to the
ground. The question FIRST suggested itself to
Newton In this form: Since the power OF gravity
extends upward above the highest point that we
can reach, why should II not extend to the moon?
And thus why might not the moon be kept In her
orbit by this very power? Knowing that the
power should vary inversely with the square OF
the distance; knowing also that the moon was
easy to calculate that the moon ought TO drop as
far in one minute as heavy body at the surface
OF the earth would fall In one second. lie there
fore proceeded 10 make this calculation, using
the supposed magnitude ut the moons orbit.
lie Inst found that the distance which the
moon dropped In one minute was less than four
teen feet. As this result did not agree with the
theory, he threw the latter aside and did not re-
turn to the calculation for nye years. He then
found that he had used wrong value of the
radius OF the earth itself. He supposed that
sixty statute miles made degree, and that the
earth was therefore only seven thousand miles
In diameter. But the more exact measures,
which had In the meantime been executed by
the French astronomers, showed that the
diameter was really eight thousand miles, and
as the distance OF the moon depended on the
OF the earth, the size OF the moons
orbit would have to be increased In the same
proportion Repeating hts calculation with
these more correct data, he found that the dis
tance which the moon dropped was really six
teen feet per minute, as required by his theory.
The next question was whether body reyoly-
ing around an attracting centre, like the planets
round the sun, the attracting force varying as
the inverse square OF the distance, would de-
scribe an ellpse having the sun In It's focus rs
the planets were known TO do AMER many al
tempts this problem was at length solved by
Newton, and the result was that this was the
very orbit In which the planet would move.
There could no longer be any doubt Of the uni
versatility Of the law which he had discovered.
In Its most comprehensive form
thus law asserts that every particle OF matter In
the universe attracts every other particle with q
which separates them.
The experiments OF Cavendish and others were
then described by the lecturer. The object Of
these experiments was to find and measure the
actual attraction exorted by heavy ball of lead.
Minute as this attraction Is, It was actually de
tected and measured by these very delicate ex-
perlments. BY the law Of grayitation, 10 IN
shown that the attraction OF globes Of matter on
bodies at their surface Is directly a. their diam
eters; and hence, ball OF the same density As
the earth, and one foot In diameter, ought to
the earths attraction. IL was actually found,
however, that the hail OF lead attracted the
body with eDe-twenty-milliont part its weight
from which 1t was concluded that the earth was
about half the density OF lead.
The lecturer then showed how the law OF
gravity was applied to the determination Of the
1Hure Of the earth, and concluded with an expo
,lii0n Of the philosophic significance OF this
greatest generalization ever made by man.