forces shall be equal.

The application of this Third Rule of Motion to planetary and celestial
phenomena is therefore philosophical, in that its conception agrees with
experience and observation.

Thus, while it is true that the sun attracts each of the planets in his
system, it is equally true that the planets, in their turn, attract the
sun with an exactly equal and opposite force. But the velocity of motion
induced by the earth's attractive power upon the sun, would be less than
the velocity of motion induced by the sun's attractive power upon the
earth, although the two forces would be equal and opposite, simply
because force, being a compound quantity, is dependent upon the mass of
a body as well as upon its velocity.

Not only, however, is it true that the sun and all the planets jointly
attract each other, but it is equally true that the planets attract each
other also, with an exactly equal and opposite effect. Indeed, as
Gravitation is universal, it has to be conceived that there are no two
bodies existing, but what the Third Law of Motion equally applies to
those two bodies; so that equality of action and re-action is as
universal as the Law of Gravitation itself.

In coming to a conclusion with reference to Philosophy and the Laws of
Motion, I wish to say that I am strongly of the opinion that the day has
come, or will soon come, when they will pass away and give place to a
more direct and simple method of working of the great Law of
Gravitation. I look upon the Laws of Motion as part of the scaffolding
which has been used to build up the Law of Gravitation. That Law has now
been erected, and stands firm and secure in its position in the
universe. Whatever changes may take place in its scaffolding, the Law
itself will stand out with greater beauty and clearness, if we could but
see the perfected structure, apart from the props and helps which have
assisted in its successful erection and completion. As Dr. Larmor said,
in his address to the British Association, 1900: "There has even
appeared a disposition to consider that the Newtonian principles, which
have formed the basis of physical phenomena for nearly two centuries,
must be replaced in these deeper subjects by a method of more direct
description of the cause of the phenomena. The question has arisen, as
to how far the new methods of aetherial physics are to be considered as
an independent departure; or how far they form the natural development
of e