XXV
b) It proves, even better than free descent of bodies, Law 4.
of falling.
c) Newton's Law of gravitation may be proved (Apparatus of
time Cavendish). If m = mass, g = gravitation, e = distance, Newton's
law may be expressed this way: g = m/e2.
d) Knowing the gravity of the earth by means of law 4 of
the pendulum, the mass of the earth may also be fixed.
e) The mass of the sun and
f) The form of the earth (137) may be determined.
g) The rotation of the earth (Poncault's experiment) can be
proved.
Remark. By the oscillations of a suspended lamp in the Cathedral of
Pisa Galileo (1564-1642) discovered the law of isochronism. Huyghens
(1660) used the pendulum at first for measuring time, and Richer (1672)
found at first that the oscillations differ with different longditude.
§ 57. 4) Curvilinear motion. It is the resultant of the
incessant co-operation of the Centrifugal and Centripetal
Force. 132.
The force, which draws the body to
wards the centre is called Centripetal force
and the other one according to which the
body tends to flee from the centre (A B)
is called Centrifugal force. Both forces
acting simultaneously the body moves in
the direction of the diagonal (Chap. II, 3.)
in No. XXIII. P A.
But as in any moment both of these forces are in action,
the body changes its direction every moment, thus moving in a
curved line, which may be supposed to consist of innumerable
little straight lines. According to the amount and direction of
both these forces the course may be a circle or spiral or an ellipse etc.
