PHYSICS
58
PHYSICS
To Gilbert we are indebted for an exhaustive trea-
tise on magnetism, in which he systematically incor-
porated what was known in medieval times of elec-
trical and magnetic phenomena, without adding
thereto anj'thing very essential; he also gave the
result of his own valuable experiments. It was in
this treatise that he began to expound his "Magnetic
Philosophy", that is to say his celestial mechanics,
but the work in which he fully developed it was not
published until 1651, long after his death. Like
Orcsme and Copernicus, Gilbert maintained that in
each star there was a particular gravity through
which the material parts belonging to this star, and
these only, tended to rejoin the star when they had
been separated from it. He comparetl this gravity,
peculiar to each star, to the action by which a piece
of iron flies towards the magnet whose nature it
shares. This opinion, held by so many of Gilbert's
predecessors and adopted by a great number of his
imitators, led Francis Bacon astray. Bacon was the
enthusiastic herald of the experimental method
which, however, he never practised and of which he
had an utterly false conception. According to Gil-
bert, the Earth, sun, and the stars were animated, and
the animating principle of each communicated to
the body the motion of perpetual rotation. From a
distance, the sun exerted an action perpendicular
to the radius vector which goes from the centre of
the sun to a planet, and this action caused the planet
to revolve around the sun just as a horse turns the
horse-mill to which it is yoked.
Kepler himself admitted that in his first attempts along the line of celestial mechanics he was under the influence of Nicholas of Cusa and Gilbert. Inspired by the former of these authors, he attributed the Earth's rotation on its axis to an impetus communi- cated by the Creator at the beginning of time; but, under the influence of Gilbert's theory, he declared that this impetus ended by being transformed into a soul or an animating principle. In Kepler's earliest system, as in Gilbert's, the distant sun was said to exercise over each planet a power perpendicular to the radius vector, which power produced the circular motion of the planet. However, Kepler had the happy thought of submitting a universal attraction for the magnetic attraction that Gilbert had con- sidered peculiar to each star. He assumed that every material mass tended towards every other material mass, no matter to what celestial body each one of them belonged; that a portion of matter placed between two stars would tend towards the larger and nearer one, although it might never have belonged to it; that, at the moment of high tide, the waters of the sea rose towards the moon, not because they had any special affinity for this humid star, but by virtue of the general tendency that draws all material masses towards one another.
In the course of numerous attempts to explain the motion of the stars, Kepler was led to complicate his first celestial mechanics. He assumed that all celestial bodies were plunged into an ethereal fluid, that the rotation of the sun engendered a vortex with- in this fluid the reactions of which interposed to deflect each planet from the circular path. He also thought that a certain power, similar to that which directs the magnetic needle, preser\'ed invariable in space the direction of the axis around which the rotation of each planet is effected. The unstable and complicated system of celestial mechanics taught by Kepler sprang from very deficient dynam- ics which, on many points, was more akin to that of the Peripatetics than to that of the Parisians. How('\er, these many vague hypotheses exerted an incontestable influence on the attempts of scientists from Kepler to Newton to (l.'tcniiine the fon-es that move the stars. If, indeed, Kepler iiicparcd the way for Newton's work, it w;us mainly by the discovery
of the three admirable laws that have immortalized
his name; and, by teaching that the planets de-
scribed ellipses instead of circles, he produced in
astronomy a revolution greater by far than that
caused by Copernicus; he destroyed the last time-
honoured principle of ancient physics, according
to which all celestial motions were reducible to cir-
cular motion.
XIX. Controversies concerning Geostatics. — The "magnetic" philosophy adopted and developed by Gilbert was not only rejected by Kepler but badly abused in a dispute over the principles of statics. A number of the Parisian Scholastics of the fourteenth century, and Albert of Saxony in particular, had accepted the principle that in every body there is a fixed, determined point which tends to join the centre of the World, this point being identical with the centre of gravity as considered by Archimedes. From this principle various authors, notably Vinci, deduced corollaries that retained a place in statics. The Copernican revolution had modified this principle but little, having simply substituted, for the centre of the universe, a particular point in each star, towards which point tended the centre of gravity of each mass belonging to this star. Copernicus, Galileo, and Gilbert admitted the prin- ciple thus modified, but Kepler rejected it. In 1635 Jean de Beaugrand deduced from this principle a paradoxical theory on the gravity of bodies, and par- ticularly on the variation in the weight of a body whose distance from the centre of the universe changes. Opinions similar to those proposed by Beaugrand in his geostatics were held in Italy by Caetelli, and in France by Pierre Fermat (1608-65). Fermat's doctrine was discussed and refuted by Etienne Pascal (1588-1651) and Gilles Persone de Roberval (1602-75), and the admirable controversy between these authors and Fermat contributed in great measure to the clear exposition of a certain number of ideas employed in statics, amongst them, that of the centre of gravity.
It was this controversy which led Descartes to revi\-e the question of virtual displacements in pre- cisely the same form as that adopted by the School of Jordanus, in order that the essential propositions of statics might be given a stable foundation. On the other hand, Torricelli based all his arguments concerning the laws of equilibrium on the axiom quoted above, viz.: a system endowed with weight is in equilibrium when the centre of gravity of all the bodies forming it is the lowest possible. Cardano and perhaps Vinci had derived this proposition from the doctrine of Albert of Saxony, but TorriceUi was care- ful to use it only under circumstances in which all verticals are considered parallel to one another and, in this way, he severed all connexion between the axiom that he admitted and the doubtful hypotheses of Parisian physics or magnetic philosophy. Thence- forth the principles of statics were formulated with accuracy, John Wallis (1616-1703), Pierre Varignon (1654-1722), and Jean Bernoulli (1667-1748) having merely to complete and develop the information pro- vided by Stevinus, Roberval, Descartes, and Tor- riceUi.
XX. Descartes's Work. — We have just stated what part Descartes took in the building of statics by bringing forward the method of virtual displace- ments, but his active interest in the building up of dynamics was still more important. He clearly for- mulated the law of inertia as observed by Benedetti: every moving body is inclined, if nothing prevent it, to continue its motion in a straight line and with constant velocity; a body cannot move in a circle unless it be drawn towards the centre, by centripetal movement in ojiposition to the centrifugal force by which this bixly temls to fly away from the centre. Because of the similarity of the views held by Des-
