Lenses and Waves

244 CHAPTER 6
From the viewpoint of mechanistic orthodoxy the introduction of
attractive forces posed, to say it mildly, a problem. In being causes external
to matter they seemed to attribute an active quality to it. Newton admitted
that he had not yet been able to deduce the cause of gravity from the
phenomena, but this did not refrain him from setting forth its properties.
“And it is enough that gravity really exists and acts according to the laws that we have
set forth and is sufficient to explain all the motions of the heavenly bodies and of our
sea.” 103
In definition four, Newton defined ‘impressed force’ descriptively:
“Impressed force is the action exerted on a body to change its state either of
resting or of moving uniformly straightforward.” 104 Forces could thus be
measured by changes in the motion of a body, as expressed in the second
law: “A change of motion is proportional to the motive force impressed and
takes place along the straight line in which that force is impressed.” 105 The
mathematical correlation of the change of motion of a satellite and of an
apple convinced Newton that the inverse square law defined gravity as a
really existing force of which the cause was yet to be discovered.
When Huygens heard of the forthcoming Principia, his response was as
might be expected. Fatio informed him in 1687 of the forthcoming
publication of a book that would change all of physics. Huygens replied that
he was curious to see the demonstrations that Fatio had sketched. “I do not
mind that he is no Cartesian as long as he does not give us suppositions like
that of attraction.” 106 Seemingly, a new Roberval had stood up across the
Channel. Upon reading Principia Huygens must have realized, however, that
this one was of different stature. Newton did derive the properties of gravity.
What is more, he derived a whole science of motion from the concept of
gravitational attraction. Here was an able mathematician treading on ground
that Huygens himself had explored earlier and on which he had acquired
fame.
What was Huygens’ reaction upon reading Principia? “Vortices destroyed
by Newton. Vortices of spherical movement in their place.” 107 In 1690, he
published – together with Traité de la Lumière – Discours de la Cause de la
Pesanteur, his 1669 paper on gravity extended with a critique of Principia.
However, in the second book Newton had demolished Descartes’ notion of
vortices. In a discussion of the motion of bodies through fluids, he had
demonstrated that a system of vortices could not explain Kepler’s laws of
planetary motion and, moreover, could not exist without some external
agent. How could Huygens believe that Principia did not refute his
explanation of gravity?
103
Newton, Principia, 943.
104
Newton, Principia, 405.
105
Newton, Principia, 416.
106
OC9, 190. “Je veux bien qu’il ne soit pas Cartesien pourveu qu’il ne nous fasse pas des suppositions
comme celle de l’attraction.”
107
OC21, 437. “Tourbillons destruits par Newton. Tourbillons de mouvement spherique a la place.”