Year: 1874

NEwToN's THEORY OF GRAvITATIoN.-
What Sir Isacc Newton did was not to
discover gravitation, for every one who
ever saw a stone fall saw an effect Of that
force. He only discovered that gravita.
ton extended into the celestial spaces.
His first reasoning was this we see that
all bodies at the surface of the earth tend
to fall toward its centre. This tendency
is seen on the highest mountains as well
as at the surface Of the ocean. How far
does it extend upward Why should it
not extend to the moon ? We know that
the moon is revolving around the earth,
and without some force to keep it in its
orbit around the earth it would fly of in.
to space, and we should soon lose sight of
it. Why shoald not this force by which
the moon is continually turned aside from
the line in which it is moving and kept
10 q path around the earth, be the same
force which causes a stone to fall to the
ground To answer these questions,
Newton began by calculating what force
would really be required to keep the moon
in her curved orbit around the earth, and
comparing it with the force which caused
heavy bodies to fall to the earth's sur
face, with the view TO learning whether
the two forces we're inversely as the
square of the distance from the earth's
centre Now allow me TO state crcum.
stance which illustrates the cxactness Of
true scientific reasoning in reaching these
conclusions. In his first calculations
Newton found that the two forces did not
correspond. Had he been any other than
man Of true science, fear he would
have twisted something so as to make
them correspond. Instead of doing SO,
he laid his theory aside for number Of
years. At length he learned that more
accurate measurements Of the size Of the
earth than those accessible to him when
he made his calculations had been execut.
ed 10y the French astronomers. and that
these gave for the size Of the earth in
English miles quantity much larger
than that which he had used in making,
his calculations. lie now found that
when he had based his computations on
the true magnitude Of the earth, the two
forces Of the stone toward the earth and
the moou in its orbit corresponded exact.
ly to the theory. We see from this that
Newton did not discover gravitation it.
self, but only that it extended into the
celestial spaces.


Not only does the moon move around
the earth but all the planets move around
the sun. If no force acted on them, they
would move forward in u straight line for


ever, and soon be lost in the celestial
spaces. The very fact that they move
in curved orbits around the sun shows
that they must be acted upon by some


force directed toward the sun The mos,
careful and exact observations that were
: made showed that this force was at all
times and in the cose of all the planet.
directed accurately toward the centre o'
the sun. Knowing the relative distance


of the various planets from the sun, il
was easy to calculate how strong thi,
fUrce must be in the c"~e or each,a and the
, result showed that in the case OF cacr


different planet its intensity was inversely J
as the planets distance from the sun. i
, remained only to show whether the clip
, tic orbit in which each planet was know


to move around the sun would be pro


duced by force inversely as the squar'
of the distance. This required mathc
, matical investigation which the scientiii'
/ men of that day found very difficult bu
, Newton at length succeeded in solving
the problem in manner which left nj


doubt 1t was now proved that each an
every planet was held in its curved orbi
, by force acting directly toward the sun
!, and being inversely as the square Of th'
distance of the planet from it. AS thi


force was to all appearances of precisel,
i the same natyre with that which mad
the moon tend toward the earth, it wa
+ quite reasonable to call it gravitation. 1
i differs from the force which makes q ston
: fall only 10 being directed toward the SUI
a instead of the earth,-From Some Talk
| Of an Astronomer" in Harper's Mago


-jue far OctoLer