An introductory text-book of logic - Mellone, Sydney - Rare Books at ...

OR SCIENTIFIC METHOD. 297
earth. But in the case of an object removed as far
from the earth as the moon is, it must have appreciably
diminished. Now the intensity of gravity at the earth s
surface is measured by the space through which a body
falls in one second ; and its intensity at the distance of
the moon, by the space through which the moon would
fall towards the earth in one second, if she were not
prevented by another cause. Also, the object at the
surface of the earth is distant from the centre of the
earth (which is the centre from which gravity acts) by
the length of the earth s radius ; and the distance of the
moon from the earth s centre is also known. Hence it be
came a calculation in Proportion, to ascertain the distance
through which the moon ought to fall towards the earth
in one second, if the Newtonian hypothesis were true.
How can we compare this result with the actual fact of
the moon s falling, since no such experiment can be tried
upon her? "Newton saw that such an experiment is in
fact constantly exhibited to us. The moon performs a
revolution in an orbit whose dimensions had been ascer
tained by astronomers ; consequently, the velocity with
which she moves was known. But this velocity im
pressed upon such a body must, if nothing else inter
fered, carry it off in a straight line through space. The
actual motion of the moon is in an orbit round the earth;
and in any given portion or arc of that orbit, the dis
tance through which, at the end of one second, the moon
has deflected from the straight line which is a tangent to
the orbit at the commencement of that second, is known.
This is the space through which the moon is actually
falling (i.e., is actually pulled)
towards the earth in one
second. Newton, then, in his calculation, had only to
take the distance of the moon from the centre of the
earth, and the distance of the surface from the centre