PHYSICS
59
PHYSICS
cartes and Bencdetti concerning this law, we may
conclude that Descartes's discoverj' was influenced
by that of Benedetti, especially as Benedetti's works
were known to Marin Mersenne (1588-1648), the
faithful friend and correspondent of Descartes.
Descartes connected the following truth with the
law of inertia: a weight constant in size and direction
causes a uniformly accelerated motion. Besides, we
have seen how, with the aid of Descartes's principles,
Gassendi was able to rectify what Galileo had taught
concerning falUng bodies and the motion of projec-
tiles.
In statics a heavj' body can very often be replaced by a material point placed at its centre of gravity; but in dynamics the question arises whether the motion of a body be treated as if this body were entirely concentrated in one of these points, and also which "point this is? This question relative to the existence and finding of a centre of impulsion had alreadv engrossed the attention of Vinci and, after him, of Bernardino Baldi (1553-1617). Baldi as- serted that, in a body undergoing a motion of trans- lation, the centre of impulsion does not differ from the centre of gravity. Xow, is there a centre of impulsion and, if so, where is it to be found in a body under- going a motion other than that of translation, for instance, by a rotation around an axis? In other words, is there a simple pendulum that moves in the same way as a given compound pendulum? Inspired, no doubt, by reading Baldi, Mer.senne laid this prob- lem before Roberval and Descartes, both of whom made great efforts to .solve it but became unfriendly to each other because of the difference in their respec- tive propositions. Of the two, Descartes came nearer to the truth, but the dynamic principles that he used were not sufficiently accurate to justify his opinion in a convincing manner; the glory was reserved to Christian Huygens.
The Jesuits, who at the College of La Fleche had been the preceptors of Mersenne and Descartes, did not teach Peripatetic phj-sics in its stereotj-ped integrity, but Parisian physics; the treatise that guided the instruction imparted at this institution being represented by the "Commentaries" on Aris- totle, published by the Jesuits of Coimbra at the close of the seventeenth centurj-. Hence it can be under- stood whj' the d}-namics of Descartes had many points in common with the dynamics of Buridan and the Parisians. Indeed, so close were the relations between Parisian and Cartesian physics that certain professors at La Fleche, such as Etienne Noel (1581- 1660), became Cartesians. Other Jesuits attempted to build up a sort of a combination of Galilean and Cartesian mechanics with the mechanics taugh by Parisian Scholasticism, and foremost among these men must be mentioned Honore Fabri (1606-88), a friend of Mersenne.
In every moving body Descartes maintained the existence of a certain power to continue its motion in the same direction and with the same velocity and this power, which he called the quantity of motion, he measured by estimating the product of the mass of the moN-ing body by the velocity that impels it. The affinity is clo.se between the role which Descartes attributed to this quantity of motion, and that which Buridan ascribed to impetus. Fabri was fully aware of this analogy and the momentum that he discussed was at once the impetus of the Parisians, and Des- cartes's quantity of motion. In statics he identified this momentum with what Galileo called momento or impelo, and this identification was certainly conform- able to the Pisan's idea. Fabri's synthesis was well adapted to make this truth clear, that modern dynam- ics, the foundations of which were laid by Descartes and Galileo, proceeded almost directly from the flynamics taught during the fourteenth century in the University of Paris.
If the special physical truths demonstrated or
anticipated by Descartes were easily traceable to the
philosophy of the fourteenth centur)-, the principles
on which the great geometrician wished to base
these truths were absolutely incompatible with this
philosophy. In fact, denj-lng that in reality there
existed anything qualitative, Descartes insisted that
matter be reduced to extension and to the attributes
of which extension seemed to him susceptible, namely,
numerical proportions and motion; and it was by
combinations of different figures and motions that
all the effects of physics could be explained according
to his Uking. Therefore the power by \-irtue of which
a body tends to preserve the direction and velocity
of its motion is not a quality distinct from motion,
such as the impetus recognized by the scholastics;
it is nothing else than the motion itself, as was taught
by WilUam of Occam at the beginning of the four-
teenth centurj-. A body in motion and isolated would
always retain the same quantity of motion, but there
is no isolated body in a vacuum, because matter being
identical with extension, vacuum is inconceivable,
as is also compressibilit}'. The onlj' conceivable
motions are those which can be produced in the midst
of incompressible matter, that is to say, vortical
motions confined within their own bulk.
In these motions bodies drive one another from the place they have occupied and, in such a transmission of motion, the quantity of motion of each of these bodies varies; however, the entire quantity of motion of all the bodies that impinge on one another remains constant, as God always maintains the same sum total of motion in the world. This transmission of motion by impact is the only action that bodies can exert over one another and in Cartesian, as well as in Aristotelean physics, a body cannot put another in motion unless it touch it, immediate action at a distance being beyond conception.
There are various species of matter, differing from one another only in the size and shape of the contig- uous particles of which they are formed. The space that extends between the different heavenly bodies is filled with a certain subtile matter, the verj' fine particles of which easily penetrate the interstices left between the coarser constituents of other bodies. The properties of subtile matter play an important part in all Cartesian cosmologj-. The vortices in which subtile matter moves, and the pressure gener- ated by these vortical motions, ser\-e to explain all celestial phenomena. Leibniz was right in supposing that for this part of his work Descartes had drawn largely upon Kepler. Descartes also strove to ex- plain, with the aid of the figures and motions of sub- tile and other matter, the different effects obser\-able in physics, particularly the properties of the magnet and of light. Light is identical with the pressure which subtile matter exerts over bodies and, as sub- tile matter is incompressible, light is instantly trans- mitted to any distance, however great.
The suppositions by the aid of which Descartes attempted to reduce all physical phenomena to com- Ijinations of figures and motions had scarcely any part in the discoveries that he made in physics; therefore the identification of fight «-ith the pressure exerted by subtile matter plays no part in the inven- tion of the new truths which Descartes taught in optics. Foremost amongst these truths is the law of the refraction of light passing from one medium to another, although the question still remains whether Descartes discovered this law himself, or whether, as Huygens accused him of doing, he borrowed it from Wiflebrord Snellius (1591-1626), without any men- tion of the real author. By this law Descartes gave the theory of refraction through a prism, which per- mitted him to measure the indices of refraction; moreover, he greatly ))erfected the study of lenses, and finally completed the explanation of the rainbow,
